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linux/kernel/rcu/tasks.h
Zqiang a4fcfbee8f rcu-tasks: Handle queue-shrink/callback-enqueue race condition 
The rcu_tasks_need_gpcb() determines whether or not: (1) There are
callbacks needing another grace period, (2) There are callbacks ready
to be invoked, and (3) It would be a good time to shrink back down to a
single-CPU callback list.  This third case is interesting because some
other CPU might be adding new callbacks, which might suddenly make this
a very bad time to be shrinking.

This is currently handled by requiring call_rcu_tasks_generic() to
enqueue callbacks under the protection of rcu_read_lock() and requiring
rcu_tasks_need_gpcb() to wait for an RCU grace period to elapse before
finalizing the transition.  This works well in practice.

Unfortunately, the current code assumes that a grace period whose end is
detected by the poll_state_synchronize_rcu() in the second "if" condition
actually ended before the earlier code counted the callbacks queued on
CPUs other than CPU 0 (local variable "ncbsnz").  Given the current code,
it is possible that a long-delayed call_rcu_tasks_generic() invocation
will queue a callback on a non-zero CPU after these CPUs have had their
callbacks counted and zero has been stored to ncbsnz.  Such a callback
would trigger the WARN_ON_ONCE() in the second "if" statement.

To see this, consider the following sequence of events:

o	CPU 0 invokes rcu_tasks_one_gp(), and counts fewer than
	rcu_task_collapse_lim callbacks.  It sees at least one
	callback queued on some other CPU, thus setting ncbsnz
	to a non-zero value.

o	CPU 1 invokes call_rcu_tasks_generic() and loads 42 from
	->percpu_enqueue_lim.  It therefore decides to enqueue its
	callback onto CPU 1's callback list, but is delayed.

o	CPU 0 sees the rcu_task_cb_adjust is non-zero and that the number
	of callbacks does not exceed rcu_task_collapse_lim.  It therefore
	checks percpu_enqueue_lim, and sees that its value is greater
	than the value one.  CPU 0 therefore  starts the shift back
	to a single callback list.  It sets ->percpu_enqueue_lim to 1,
	but CPU 1 has already read the old value of 42.  It also gets
	a grace-period state value from get_state_synchronize_rcu().

o	CPU 0 sees that ncbsnz is non-zero in its second "if" statement,
	so it declines to finalize the shrink operation.

o	CPU 0 again invokes rcu_tasks_one_gp(), and counts fewer than
	rcu_task_collapse_lim callbacks.  It also sees that there are
	no callback queued on any other CPU, and thus sets ncbsnz to zero.

o	CPU 1 resumes execution and enqueues its callback onto its own
	list.  This invalidates the value of ncbsnz.

o	CPU 0 sees the rcu_task_cb_adjust is non-zero and that the number
	of callbacks does not exceed rcu_task_collapse_lim.  It therefore
	checks percpu_enqueue_lim, but sees that its value is already
	unity.	It therefore does not get a new grace-period state value.

o	CPU 0 sees that rcu_task_cb_adjust is non-zero, ncbsnz is zero,
	and that poll_state_synchronize_rcu() says that the grace period
	has completed.  it therefore finalizes the shrink operation,
	setting ->percpu_dequeue_lim to the value one.

o	CPU 0 does a debug check, scanning the other CPUs' callback lists.
	It sees that CPU 1's list has a callback, so it (rightly)
	triggers the WARN_ON_ONCE().  After all, the new value of
	->percpu_dequeue_lim says to not bother looking at CPU 1's
	callback list, which means that this callback will never be
	invoked.  This can result in hangs and maybe even OOMs.

Based on long experience with rcutorture, this is an extremely
low-probability race condition, but it really can happen, especially in
preemptible kernels or within guest OSes.

This commit therefore checks for completion of the grace period
before counting callbacks.  With this change, in the above failure
scenario CPU 0 would know not to prematurely end the shrink operation
because the grace period would not have completed before the count
operation started.

[ paulmck: Adjust grace-period end rather than adding RCU reader. ]
[ paulmck: Avoid spurious WARN_ON_ONCE() with ->percpu_dequeue_lim check. ]

Signed-off-by: Zqiang <qiang1.zhang@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
2023-01-03 17:52:17 -08:00

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/* SPDX-License-Identifier: GPL-2.0+ */
/*
* Task-based RCU implementations.
*
* Copyright (C) 2020 Paul E. McKenney
*/
#ifdef CONFIG_TASKS_RCU_GENERIC
#include "rcu_segcblist.h"
////////////////////////////////////////////////////////////////////////
//
// Generic data structures.
struct rcu_tasks;
typedef void (*rcu_tasks_gp_func_t)(struct rcu_tasks *rtp);
typedef void (*pregp_func_t)(struct list_head *hop);
typedef void (*pertask_func_t)(struct task_struct *t, struct list_head *hop);
typedef void (*postscan_func_t)(struct list_head *hop);
typedef void (*holdouts_func_t)(struct list_head *hop, bool ndrpt, bool *frptp);
typedef void (*postgp_func_t)(struct rcu_tasks *rtp);
/**
* struct rcu_tasks_percpu - Per-CPU component of definition for a Tasks-RCU-like mechanism.
* @cblist: Callback list.
* @lock: Lock protecting per-CPU callback list.
* @rtp_jiffies: Jiffies counter value for statistics.
* @rtp_n_lock_retries: Rough lock-contention statistic.
* @rtp_work: Work queue for invoking callbacks.
* @rtp_irq_work: IRQ work queue for deferred wakeups.
* @barrier_q_head: RCU callback for barrier operation.
* @rtp_blkd_tasks: List of tasks blocked as readers.
* @cpu: CPU number corresponding to this entry.
* @rtpp: Pointer to the rcu_tasks structure.
*/
struct rcu_tasks_percpu {
struct rcu_segcblist cblist;
raw_spinlock_t __private lock;
unsigned long rtp_jiffies;
unsigned long rtp_n_lock_retries;
struct work_struct rtp_work;
struct irq_work rtp_irq_work;
struct rcu_head barrier_q_head;
struct list_head rtp_blkd_tasks;
int cpu;
struct rcu_tasks *rtpp;
};
/**
* struct rcu_tasks - Definition for a Tasks-RCU-like mechanism.
* @cbs_wait: RCU wait allowing a new callback to get kthread's attention.
* @cbs_gbl_lock: Lock protecting callback list.
* @tasks_gp_mutex: Mutex protecting grace period, needed during mid-boot dead zone.
* @kthread_ptr: This flavor's grace-period/callback-invocation kthread.
* @gp_func: This flavor's grace-period-wait function.
* @gp_state: Grace period's most recent state transition (debugging).
* @gp_sleep: Per-grace-period sleep to prevent CPU-bound looping.
* @init_fract: Initial backoff sleep interval.
* @gp_jiffies: Time of last @gp_state transition.
* @gp_start: Most recent grace-period start in jiffies.
* @tasks_gp_seq: Number of grace periods completed since boot.
* @n_ipis: Number of IPIs sent to encourage grace periods to end.
* @n_ipis_fails: Number of IPI-send failures.
* @pregp_func: This flavor's pre-grace-period function (optional).
* @pertask_func: This flavor's per-task scan function (optional).
* @postscan_func: This flavor's post-task scan function (optional).
* @holdouts_func: This flavor's holdout-list scan function (optional).
* @postgp_func: This flavor's post-grace-period function (optional).
* @call_func: This flavor's call_rcu()-equivalent function.
* @rtpcpu: This flavor's rcu_tasks_percpu structure.
* @percpu_enqueue_shift: Shift down CPU ID this much when enqueuing callbacks.
* @percpu_enqueue_lim: Number of per-CPU callback queues in use for enqueuing.
* @percpu_dequeue_lim: Number of per-CPU callback queues in use for dequeuing.
* @percpu_dequeue_gpseq: RCU grace-period number to propagate enqueue limit to dequeuers.
* @barrier_q_mutex: Serialize barrier operations.
* @barrier_q_count: Number of queues being waited on.
* @barrier_q_completion: Barrier wait/wakeup mechanism.
* @barrier_q_seq: Sequence number for barrier operations.
* @name: This flavor's textual name.
* @kname: This flavor's kthread name.
*/
struct rcu_tasks {
struct rcuwait cbs_wait;
raw_spinlock_t cbs_gbl_lock;
struct mutex tasks_gp_mutex;
int gp_state;
int gp_sleep;
int init_fract;
unsigned long gp_jiffies;
unsigned long gp_start;
unsigned long tasks_gp_seq;
unsigned long n_ipis;
unsigned long n_ipis_fails;
struct task_struct *kthread_ptr;
rcu_tasks_gp_func_t gp_func;
pregp_func_t pregp_func;
pertask_func_t pertask_func;
postscan_func_t postscan_func;
holdouts_func_t holdouts_func;
postgp_func_t postgp_func;
call_rcu_func_t call_func;
struct rcu_tasks_percpu __percpu *rtpcpu;
int percpu_enqueue_shift;
int percpu_enqueue_lim;
int percpu_dequeue_lim;
unsigned long percpu_dequeue_gpseq;
struct mutex barrier_q_mutex;
atomic_t barrier_q_count;
struct completion barrier_q_completion;
unsigned long barrier_q_seq;
char *name;
char *kname;
};
static void call_rcu_tasks_iw_wakeup(struct irq_work *iwp);
#define DEFINE_RCU_TASKS(rt_name, gp, call, n) \
static DEFINE_PER_CPU(struct rcu_tasks_percpu, rt_name ## __percpu) = { \
.lock = __RAW_SPIN_LOCK_UNLOCKED(rt_name ## __percpu.cbs_pcpu_lock), \
.rtp_irq_work = IRQ_WORK_INIT_HARD(call_rcu_tasks_iw_wakeup), \
}; \
static struct rcu_tasks rt_name = \
{ \
.cbs_wait = __RCUWAIT_INITIALIZER(rt_name.wait), \
.cbs_gbl_lock = __RAW_SPIN_LOCK_UNLOCKED(rt_name.cbs_gbl_lock), \
.tasks_gp_mutex = __MUTEX_INITIALIZER(rt_name.tasks_gp_mutex), \
.gp_func = gp, \
.call_func = call, \
.rtpcpu = &rt_name ## __percpu, \
.name = n, \
.percpu_enqueue_shift = order_base_2(CONFIG_NR_CPUS), \
.percpu_enqueue_lim = 1, \
.percpu_dequeue_lim = 1, \
.barrier_q_mutex = __MUTEX_INITIALIZER(rt_name.barrier_q_mutex), \
.barrier_q_seq = (0UL - 50UL) << RCU_SEQ_CTR_SHIFT, \
.kname = #rt_name, \
}
/* Track exiting tasks in order to allow them to be waited for. */
DEFINE_STATIC_SRCU(tasks_rcu_exit_srcu);
/* Avoid IPIing CPUs early in the grace period. */
#define RCU_TASK_IPI_DELAY (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB) ? HZ / 2 : 0)
static int rcu_task_ipi_delay __read_mostly = RCU_TASK_IPI_DELAY;
module_param(rcu_task_ipi_delay, int, 0644);
/* Control stall timeouts. Disable with <= 0, otherwise jiffies till stall. */
#define RCU_TASK_BOOT_STALL_TIMEOUT (HZ * 30)
#define RCU_TASK_STALL_TIMEOUT (HZ * 60 * 10)
static int rcu_task_stall_timeout __read_mostly = RCU_TASK_STALL_TIMEOUT;
module_param(rcu_task_stall_timeout, int, 0644);
#define RCU_TASK_STALL_INFO (HZ * 10)
static int rcu_task_stall_info __read_mostly = RCU_TASK_STALL_INFO;
module_param(rcu_task_stall_info, int, 0644);
static int rcu_task_stall_info_mult __read_mostly = 3;
module_param(rcu_task_stall_info_mult, int, 0444);
static int rcu_task_enqueue_lim __read_mostly = -1;
module_param(rcu_task_enqueue_lim, int, 0444);
static bool rcu_task_cb_adjust;
static int rcu_task_contend_lim __read_mostly = 100;
module_param(rcu_task_contend_lim, int, 0444);
static int rcu_task_collapse_lim __read_mostly = 10;
module_param(rcu_task_collapse_lim, int, 0444);
/* RCU tasks grace-period state for debugging. */
#define RTGS_INIT 0
#define RTGS_WAIT_WAIT_CBS 1
#define RTGS_WAIT_GP 2
#define RTGS_PRE_WAIT_GP 3
#define RTGS_SCAN_TASKLIST 4
#define RTGS_POST_SCAN_TASKLIST 5
#define RTGS_WAIT_SCAN_HOLDOUTS 6
#define RTGS_SCAN_HOLDOUTS 7
#define RTGS_POST_GP 8
#define RTGS_WAIT_READERS 9
#define RTGS_INVOKE_CBS 10
#define RTGS_WAIT_CBS 11
#ifndef CONFIG_TINY_RCU
static const char * const rcu_tasks_gp_state_names[] = {
"RTGS_INIT",
"RTGS_WAIT_WAIT_CBS",
"RTGS_WAIT_GP",
"RTGS_PRE_WAIT_GP",
"RTGS_SCAN_TASKLIST",
"RTGS_POST_SCAN_TASKLIST",
"RTGS_WAIT_SCAN_HOLDOUTS",
"RTGS_SCAN_HOLDOUTS",
"RTGS_POST_GP",
"RTGS_WAIT_READERS",
"RTGS_INVOKE_CBS",
"RTGS_WAIT_CBS",
};
#endif /* #ifndef CONFIG_TINY_RCU */
////////////////////////////////////////////////////////////////////////
//
// Generic code.
static void rcu_tasks_invoke_cbs_wq(struct work_struct *wp);
/* Record grace-period phase and time. */
static void set_tasks_gp_state(struct rcu_tasks *rtp, int newstate)
{
rtp->gp_state = newstate;
rtp->gp_jiffies = jiffies;
}
#ifndef CONFIG_TINY_RCU
/* Return state name. */
static const char *tasks_gp_state_getname(struct rcu_tasks *rtp)
{
int i = data_race(rtp->gp_state); // Let KCSAN detect update races
int j = READ_ONCE(i); // Prevent the compiler from reading twice
if (j >= ARRAY_SIZE(rcu_tasks_gp_state_names))
return "???";
return rcu_tasks_gp_state_names[j];
}
#endif /* #ifndef CONFIG_TINY_RCU */
// Initialize per-CPU callback lists for the specified flavor of
// Tasks RCU.
static void cblist_init_generic(struct rcu_tasks *rtp)
{
int cpu;
unsigned long flags;
int lim;
int shift;
raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
if (rcu_task_enqueue_lim < 0) {
rcu_task_enqueue_lim = 1;
rcu_task_cb_adjust = true;
pr_info("%s: Setting adjustable number of callback queues.\n", __func__);
} else if (rcu_task_enqueue_lim == 0) {
rcu_task_enqueue_lim = 1;
}
lim = rcu_task_enqueue_lim;
if (lim > nr_cpu_ids)
lim = nr_cpu_ids;
shift = ilog2(nr_cpu_ids / lim);
if (((nr_cpu_ids - 1) >> shift) >= lim)
shift++;
WRITE_ONCE(rtp->percpu_enqueue_shift, shift);
WRITE_ONCE(rtp->percpu_dequeue_lim, lim);
smp_store_release(&rtp->percpu_enqueue_lim, lim);
for_each_possible_cpu(cpu) {
struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
WARN_ON_ONCE(!rtpcp);
if (cpu)
raw_spin_lock_init(&ACCESS_PRIVATE(rtpcp, lock));
raw_spin_lock_rcu_node(rtpcp); // irqs already disabled.
if (rcu_segcblist_empty(&rtpcp->cblist))
rcu_segcblist_init(&rtpcp->cblist);
INIT_WORK(&rtpcp->rtp_work, rcu_tasks_invoke_cbs_wq);
rtpcp->cpu = cpu;
rtpcp->rtpp = rtp;
if (!rtpcp->rtp_blkd_tasks.next)
INIT_LIST_HEAD(&rtpcp->rtp_blkd_tasks);
raw_spin_unlock_rcu_node(rtpcp); // irqs remain disabled.
}
raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
pr_info("%s: Setting shift to %d and lim to %d.\n", __func__, data_race(rtp->percpu_enqueue_shift), data_race(rtp->percpu_enqueue_lim));
}
// IRQ-work handler that does deferred wakeup for call_rcu_tasks_generic().
static void call_rcu_tasks_iw_wakeup(struct irq_work *iwp)
{
struct rcu_tasks *rtp;
struct rcu_tasks_percpu *rtpcp = container_of(iwp, struct rcu_tasks_percpu, rtp_irq_work);
rtp = rtpcp->rtpp;
rcuwait_wake_up(&rtp->cbs_wait);
}
// Enqueue a callback for the specified flavor of Tasks RCU.
static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t func,
struct rcu_tasks *rtp)
{
int chosen_cpu;
unsigned long flags;
int ideal_cpu;
unsigned long j;
bool needadjust = false;
bool needwake;
struct rcu_tasks_percpu *rtpcp;
rhp->next = NULL;
rhp->func = func;
local_irq_save(flags);
rcu_read_lock();
ideal_cpu = smp_processor_id() >> READ_ONCE(rtp->percpu_enqueue_shift);
chosen_cpu = cpumask_next(ideal_cpu - 1, cpu_possible_mask);
rtpcp = per_cpu_ptr(rtp->rtpcpu, chosen_cpu);
if (!raw_spin_trylock_rcu_node(rtpcp)) { // irqs already disabled.
raw_spin_lock_rcu_node(rtpcp); // irqs already disabled.
j = jiffies;
if (rtpcp->rtp_jiffies != j) {
rtpcp->rtp_jiffies = j;
rtpcp->rtp_n_lock_retries = 0;
}
if (rcu_task_cb_adjust && ++rtpcp->rtp_n_lock_retries > rcu_task_contend_lim &&
READ_ONCE(rtp->percpu_enqueue_lim) != nr_cpu_ids)
needadjust = true; // Defer adjustment to avoid deadlock.
}
if (!rcu_segcblist_is_enabled(&rtpcp->cblist)) {
raw_spin_unlock_rcu_node(rtpcp); // irqs remain disabled.
cblist_init_generic(rtp);
raw_spin_lock_rcu_node(rtpcp); // irqs already disabled.
}
needwake = rcu_segcblist_empty(&rtpcp->cblist);
rcu_segcblist_enqueue(&rtpcp->cblist, rhp);
raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
if (unlikely(needadjust)) {
raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
if (rtp->percpu_enqueue_lim != nr_cpu_ids) {
WRITE_ONCE(rtp->percpu_enqueue_shift, 0);
WRITE_ONCE(rtp->percpu_dequeue_lim, nr_cpu_ids);
smp_store_release(&rtp->percpu_enqueue_lim, nr_cpu_ids);
pr_info("Switching %s to per-CPU callback queuing.\n", rtp->name);
}
raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
}
rcu_read_unlock();
/* We can't create the thread unless interrupts are enabled. */
if (needwake && READ_ONCE(rtp->kthread_ptr))
irq_work_queue(&rtpcp->rtp_irq_work);
}
// RCU callback function for rcu_barrier_tasks_generic().
static void rcu_barrier_tasks_generic_cb(struct rcu_head *rhp)
{
struct rcu_tasks *rtp;
struct rcu_tasks_percpu *rtpcp;
rtpcp = container_of(rhp, struct rcu_tasks_percpu, barrier_q_head);
rtp = rtpcp->rtpp;
if (atomic_dec_and_test(&rtp->barrier_q_count))
complete(&rtp->barrier_q_completion);
}
// Wait for all in-flight callbacks for the specified RCU Tasks flavor.
// Operates in a manner similar to rcu_barrier().
static void rcu_barrier_tasks_generic(struct rcu_tasks *rtp)
{
int cpu;
unsigned long flags;
struct rcu_tasks_percpu *rtpcp;
unsigned long s = rcu_seq_snap(&rtp->barrier_q_seq);
mutex_lock(&rtp->barrier_q_mutex);
if (rcu_seq_done(&rtp->barrier_q_seq, s)) {
smp_mb();
mutex_unlock(&rtp->barrier_q_mutex);
return;
}
rcu_seq_start(&rtp->barrier_q_seq);
init_completion(&rtp->barrier_q_completion);
atomic_set(&rtp->barrier_q_count, 2);
for_each_possible_cpu(cpu) {
if (cpu >= smp_load_acquire(&rtp->percpu_dequeue_lim))
break;
rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
rtpcp->barrier_q_head.func = rcu_barrier_tasks_generic_cb;
raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
if (rcu_segcblist_entrain(&rtpcp->cblist, &rtpcp->barrier_q_head))
atomic_inc(&rtp->barrier_q_count);
raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
}
if (atomic_sub_and_test(2, &rtp->barrier_q_count))
complete(&rtp->barrier_q_completion);
wait_for_completion(&rtp->barrier_q_completion);
rcu_seq_end(&rtp->barrier_q_seq);
mutex_unlock(&rtp->barrier_q_mutex);
}
// Advance callbacks and indicate whether either a grace period or
// callback invocation is needed.
static int rcu_tasks_need_gpcb(struct rcu_tasks *rtp)
{
int cpu;
unsigned long flags;
bool gpdone = poll_state_synchronize_rcu(rtp->percpu_dequeue_gpseq);
long n;
long ncbs = 0;
long ncbsnz = 0;
int needgpcb = 0;
for (cpu = 0; cpu < smp_load_acquire(&rtp->percpu_dequeue_lim); cpu++) {
struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
/* Advance and accelerate any new callbacks. */
if (!rcu_segcblist_n_cbs(&rtpcp->cblist))
continue;
raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
// Should we shrink down to a single callback queue?
n = rcu_segcblist_n_cbs(&rtpcp->cblist);
if (n) {
ncbs += n;
if (cpu > 0)
ncbsnz += n;
}
rcu_segcblist_advance(&rtpcp->cblist, rcu_seq_current(&rtp->tasks_gp_seq));
(void)rcu_segcblist_accelerate(&rtpcp->cblist, rcu_seq_snap(&rtp->tasks_gp_seq));
if (rcu_segcblist_pend_cbs(&rtpcp->cblist))
needgpcb |= 0x3;
if (!rcu_segcblist_empty(&rtpcp->cblist))
needgpcb |= 0x1;
raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
}
// Shrink down to a single callback queue if appropriate.
// This is done in two stages: (1) If there are no more than
// rcu_task_collapse_lim callbacks on CPU 0 and none on any other
// CPU, limit enqueueing to CPU 0. (2) After an RCU grace period,
// if there has not been an increase in callbacks, limit dequeuing
// to CPU 0. Note the matching RCU read-side critical section in
// call_rcu_tasks_generic().
if (rcu_task_cb_adjust && ncbs <= rcu_task_collapse_lim) {
raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
if (rtp->percpu_enqueue_lim > 1) {
WRITE_ONCE(rtp->percpu_enqueue_shift, order_base_2(nr_cpu_ids));
smp_store_release(&rtp->percpu_enqueue_lim, 1);
rtp->percpu_dequeue_gpseq = get_state_synchronize_rcu();
gpdone = false;
pr_info("Starting switch %s to CPU-0 callback queuing.\n", rtp->name);
}
raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
}
if (rcu_task_cb_adjust && !ncbsnz && gpdone) {
raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
if (rtp->percpu_enqueue_lim < rtp->percpu_dequeue_lim) {
WRITE_ONCE(rtp->percpu_dequeue_lim, 1);
pr_info("Completing switch %s to CPU-0 callback queuing.\n", rtp->name);
}
if (rtp->percpu_dequeue_lim == 1) {
for (cpu = rtp->percpu_dequeue_lim; cpu < nr_cpu_ids; cpu++) {
struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
WARN_ON_ONCE(rcu_segcblist_n_cbs(&rtpcp->cblist));
}
}
raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
}
return needgpcb;
}
// Advance callbacks and invoke any that are ready.
static void rcu_tasks_invoke_cbs(struct rcu_tasks *rtp, struct rcu_tasks_percpu *rtpcp)
{
int cpu;
int cpunext;
unsigned long flags;
int len;
struct rcu_head *rhp;
struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl);
struct rcu_tasks_percpu *rtpcp_next;
cpu = rtpcp->cpu;
cpunext = cpu * 2 + 1;
if (cpunext < smp_load_acquire(&rtp->percpu_dequeue_lim)) {
rtpcp_next = per_cpu_ptr(rtp->rtpcpu, cpunext);
queue_work_on(cpunext, system_wq, &rtpcp_next->rtp_work);
cpunext++;
if (cpunext < smp_load_acquire(&rtp->percpu_dequeue_lim)) {
rtpcp_next = per_cpu_ptr(rtp->rtpcpu, cpunext);
queue_work_on(cpunext, system_wq, &rtpcp_next->rtp_work);
}
}
if (rcu_segcblist_empty(&rtpcp->cblist) || !cpu_possible(cpu))
return;
raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
rcu_segcblist_advance(&rtpcp->cblist, rcu_seq_current(&rtp->tasks_gp_seq));
rcu_segcblist_extract_done_cbs(&rtpcp->cblist, &rcl);
raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
len = rcl.len;
for (rhp = rcu_cblist_dequeue(&rcl); rhp; rhp = rcu_cblist_dequeue(&rcl)) {
local_bh_disable();
rhp->func(rhp);
local_bh_enable();
cond_resched();
}
raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
rcu_segcblist_add_len(&rtpcp->cblist, -len);
(void)rcu_segcblist_accelerate(&rtpcp->cblist, rcu_seq_snap(&rtp->tasks_gp_seq));
raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
}
// Workqueue flood to advance callbacks and invoke any that are ready.
static void rcu_tasks_invoke_cbs_wq(struct work_struct *wp)
{
struct rcu_tasks *rtp;
struct rcu_tasks_percpu *rtpcp = container_of(wp, struct rcu_tasks_percpu, rtp_work);
rtp = rtpcp->rtpp;
rcu_tasks_invoke_cbs(rtp, rtpcp);
}
// Wait for one grace period.
static void rcu_tasks_one_gp(struct rcu_tasks *rtp, bool midboot)
{
int needgpcb;
mutex_lock(&rtp->tasks_gp_mutex);
// If there were none, wait a bit and start over.
if (unlikely(midboot)) {
needgpcb = 0x2;
} else {
set_tasks_gp_state(rtp, RTGS_WAIT_CBS);
rcuwait_wait_event(&rtp->cbs_wait,
(needgpcb = rcu_tasks_need_gpcb(rtp)),
TASK_IDLE);
}
if (needgpcb & 0x2) {
// Wait for one grace period.
set_tasks_gp_state(rtp, RTGS_WAIT_GP);
rtp->gp_start = jiffies;
rcu_seq_start(&rtp->tasks_gp_seq);
rtp->gp_func(rtp);
rcu_seq_end(&rtp->tasks_gp_seq);
}
// Invoke callbacks.
set_tasks_gp_state(rtp, RTGS_INVOKE_CBS);
rcu_tasks_invoke_cbs(rtp, per_cpu_ptr(rtp->rtpcpu, 0));
mutex_unlock(&rtp->tasks_gp_mutex);
}
// RCU-tasks kthread that detects grace periods and invokes callbacks.
static int __noreturn rcu_tasks_kthread(void *arg)
{
struct rcu_tasks *rtp = arg;
/* Run on housekeeping CPUs by default. Sysadm can move if desired. */
housekeeping_affine(current, HK_TYPE_RCU);
WRITE_ONCE(rtp->kthread_ptr, current); // Let GPs start!
/*
* Each pass through the following loop makes one check for
* newly arrived callbacks, and, if there are some, waits for
* one RCU-tasks grace period and then invokes the callbacks.
* This loop is terminated by the system going down. ;-)
*/
for (;;) {
// Wait for one grace period and invoke any callbacks
// that are ready.
rcu_tasks_one_gp(rtp, false);
// Paranoid sleep to keep this from entering a tight loop.
schedule_timeout_idle(rtp->gp_sleep);
}
}
// Wait for a grace period for the specified flavor of Tasks RCU.
static void synchronize_rcu_tasks_generic(struct rcu_tasks *rtp)
{
/* Complain if the scheduler has not started. */
if (WARN_ONCE(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE,
"synchronize_%s() called too soon", rtp->name))
return;
// If the grace-period kthread is running, use it.
if (READ_ONCE(rtp->kthread_ptr)) {
wait_rcu_gp(rtp->call_func);
return;
}
rcu_tasks_one_gp(rtp, true);
}
/* Spawn RCU-tasks grace-period kthread. */
static void __init rcu_spawn_tasks_kthread_generic(struct rcu_tasks *rtp)
{
struct task_struct *t;
t = kthread_run(rcu_tasks_kthread, rtp, "%s_kthread", rtp->kname);
if (WARN_ONCE(IS_ERR(t), "%s: Could not start %s grace-period kthread, OOM is now expected behavior\n", __func__, rtp->name))
return;
smp_mb(); /* Ensure others see full kthread. */
}
#ifndef CONFIG_TINY_RCU
/*
* Print any non-default Tasks RCU settings.
*/
static void __init rcu_tasks_bootup_oddness(void)
{
#if defined(CONFIG_TASKS_RCU) || defined(CONFIG_TASKS_TRACE_RCU)
int rtsimc;
if (rcu_task_stall_timeout != RCU_TASK_STALL_TIMEOUT)
pr_info("\tTasks-RCU CPU stall warnings timeout set to %d (rcu_task_stall_timeout).\n", rcu_task_stall_timeout);
rtsimc = clamp(rcu_task_stall_info_mult, 1, 10);
if (rtsimc != rcu_task_stall_info_mult) {
pr_info("\tTasks-RCU CPU stall info multiplier clamped to %d (rcu_task_stall_info_mult).\n", rtsimc);
rcu_task_stall_info_mult = rtsimc;
}
#endif /* #ifdef CONFIG_TASKS_RCU */
#ifdef CONFIG_TASKS_RCU
pr_info("\tTrampoline variant of Tasks RCU enabled.\n");
#endif /* #ifdef CONFIG_TASKS_RCU */
#ifdef CONFIG_TASKS_RUDE_RCU
pr_info("\tRude variant of Tasks RCU enabled.\n");
#endif /* #ifdef CONFIG_TASKS_RUDE_RCU */
#ifdef CONFIG_TASKS_TRACE_RCU
pr_info("\tTracing variant of Tasks RCU enabled.\n");
#endif /* #ifdef CONFIG_TASKS_TRACE_RCU */
}
#endif /* #ifndef CONFIG_TINY_RCU */
#ifndef CONFIG_TINY_RCU
/* Dump out rcutorture-relevant state common to all RCU-tasks flavors. */
static void show_rcu_tasks_generic_gp_kthread(struct rcu_tasks *rtp, char *s)
{
int cpu;
bool havecbs = false;
for_each_possible_cpu(cpu) {
struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
if (!data_race(rcu_segcblist_empty(&rtpcp->cblist))) {
havecbs = true;
break;
}
}
pr_info("%s: %s(%d) since %lu g:%lu i:%lu/%lu %c%c %s\n",
rtp->kname,
tasks_gp_state_getname(rtp), data_race(rtp->gp_state),
jiffies - data_race(rtp->gp_jiffies),
data_race(rcu_seq_current(&rtp->tasks_gp_seq)),
data_race(rtp->n_ipis_fails), data_race(rtp->n_ipis),
".k"[!!data_race(rtp->kthread_ptr)],
".C"[havecbs],
s);
}
#endif // #ifndef CONFIG_TINY_RCU
static void exit_tasks_rcu_finish_trace(struct task_struct *t);
#if defined(CONFIG_TASKS_RCU) || defined(CONFIG_TASKS_TRACE_RCU)
////////////////////////////////////////////////////////////////////////
//
// Shared code between task-list-scanning variants of Tasks RCU.
/* Wait for one RCU-tasks grace period. */
static void rcu_tasks_wait_gp(struct rcu_tasks *rtp)
{
struct task_struct *g;
int fract;
LIST_HEAD(holdouts);
unsigned long j;
unsigned long lastinfo;
unsigned long lastreport;
bool reported = false;
int rtsi;
struct task_struct *t;
set_tasks_gp_state(rtp, RTGS_PRE_WAIT_GP);
rtp->pregp_func(&holdouts);
/*
* There were callbacks, so we need to wait for an RCU-tasks
* grace period. Start off by scanning the task list for tasks
* that are not already voluntarily blocked. Mark these tasks
* and make a list of them in holdouts.
*/
set_tasks_gp_state(rtp, RTGS_SCAN_TASKLIST);
if (rtp->pertask_func) {
rcu_read_lock();
for_each_process_thread(g, t)
rtp->pertask_func(t, &holdouts);
rcu_read_unlock();
}
set_tasks_gp_state(rtp, RTGS_POST_SCAN_TASKLIST);
rtp->postscan_func(&holdouts);
/*
* Each pass through the following loop scans the list of holdout
* tasks, removing any that are no longer holdouts. When the list
* is empty, we are done.
*/
lastreport = jiffies;
lastinfo = lastreport;
rtsi = READ_ONCE(rcu_task_stall_info);
// Start off with initial wait and slowly back off to 1 HZ wait.
fract = rtp->init_fract;
while (!list_empty(&holdouts)) {
ktime_t exp;
bool firstreport;
bool needreport;
int rtst;
// Slowly back off waiting for holdouts
set_tasks_gp_state(rtp, RTGS_WAIT_SCAN_HOLDOUTS);
if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
schedule_timeout_idle(fract);
} else {
exp = jiffies_to_nsecs(fract);
__set_current_state(TASK_IDLE);
schedule_hrtimeout_range(&exp, jiffies_to_nsecs(HZ / 2), HRTIMER_MODE_REL_HARD);
}
if (fract < HZ)
fract++;
rtst = READ_ONCE(rcu_task_stall_timeout);
needreport = rtst > 0 && time_after(jiffies, lastreport + rtst);
if (needreport) {
lastreport = jiffies;
reported = true;
}
firstreport = true;
WARN_ON(signal_pending(current));
set_tasks_gp_state(rtp, RTGS_SCAN_HOLDOUTS);
rtp->holdouts_func(&holdouts, needreport, &firstreport);
// Print pre-stall informational messages if needed.
j = jiffies;
if (rtsi > 0 && !reported && time_after(j, lastinfo + rtsi)) {
lastinfo = j;
rtsi = rtsi * rcu_task_stall_info_mult;
pr_info("%s: %s grace period number %lu (since boot) is %lu jiffies old.\n",
__func__, rtp->kname, rtp->tasks_gp_seq, j - rtp->gp_start);
}
}
set_tasks_gp_state(rtp, RTGS_POST_GP);
rtp->postgp_func(rtp);
}
#endif /* #if defined(CONFIG_TASKS_RCU) || defined(CONFIG_TASKS_TRACE_RCU) */
#ifdef CONFIG_TASKS_RCU
////////////////////////////////////////////////////////////////////////
//
// Simple variant of RCU whose quiescent states are voluntary context
// switch, cond_resched_tasks_rcu_qs(), user-space execution, and idle.
// As such, grace periods can take one good long time. There are no
// read-side primitives similar to rcu_read_lock() and rcu_read_unlock()
// because this implementation is intended to get the system into a safe
// state for some of the manipulations involved in tracing and the like.
// Finally, this implementation does not support high call_rcu_tasks()
// rates from multiple CPUs. If this is required, per-CPU callback lists
// will be needed.
//
// The implementation uses rcu_tasks_wait_gp(), which relies on function
// pointers in the rcu_tasks structure. The rcu_spawn_tasks_kthread()
// function sets these function pointers up so that rcu_tasks_wait_gp()
// invokes these functions in this order:
//
// rcu_tasks_pregp_step():
// Invokes synchronize_rcu() in order to wait for all in-flight
// t->on_rq and t->nvcsw transitions to complete. This works because
// all such transitions are carried out with interrupts disabled.
// rcu_tasks_pertask(), invoked on every non-idle task:
// For every runnable non-idle task other than the current one, use
// get_task_struct() to pin down that task, snapshot that task's
// number of voluntary context switches, and add that task to the
// holdout list.
// rcu_tasks_postscan():
// Invoke synchronize_srcu() to ensure that all tasks that were
// in the process of exiting (and which thus might not know to
// synchronize with this RCU Tasks grace period) have completed
// exiting.
// check_all_holdout_tasks(), repeatedly until holdout list is empty:
// Scans the holdout list, attempting to identify a quiescent state
// for each task on the list. If there is a quiescent state, the
// corresponding task is removed from the holdout list.
// rcu_tasks_postgp():
// Invokes synchronize_rcu() in order to ensure that all prior
// t->on_rq and t->nvcsw transitions are seen by all CPUs and tasks
// to have happened before the end of this RCU Tasks grace period.
// Again, this works because all such transitions are carried out
// with interrupts disabled.
//
// For each exiting task, the exit_tasks_rcu_start() and
// exit_tasks_rcu_finish() functions begin and end, respectively, the SRCU
// read-side critical sections waited for by rcu_tasks_postscan().
//
// Pre-grace-period update-side code is ordered before the grace
// via the raw_spin_lock.*rcu_node(). Pre-grace-period read-side code
// is ordered before the grace period via synchronize_rcu() call in
// rcu_tasks_pregp_step() and by the scheduler's locks and interrupt
// disabling.
/* Pre-grace-period preparation. */
static void rcu_tasks_pregp_step(struct list_head *hop)
{
/*
* Wait for all pre-existing t->on_rq and t->nvcsw transitions
* to complete. Invoking synchronize_rcu() suffices because all
* these transitions occur with interrupts disabled. Without this
* synchronize_rcu(), a read-side critical section that started
* before the grace period might be incorrectly seen as having
* started after the grace period.
*
* This synchronize_rcu() also dispenses with the need for a
* memory barrier on the first store to t->rcu_tasks_holdout,
* as it forces the store to happen after the beginning of the
* grace period.
*/
synchronize_rcu();
}
/* Per-task initial processing. */
static void rcu_tasks_pertask(struct task_struct *t, struct list_head *hop)
{
if (t != current && READ_ONCE(t->on_rq) && !is_idle_task(t)) {
get_task_struct(t);
t->rcu_tasks_nvcsw = READ_ONCE(t->nvcsw);
WRITE_ONCE(t->rcu_tasks_holdout, true);
list_add(&t->rcu_tasks_holdout_list, hop);
}
}
/* Processing between scanning taskslist and draining the holdout list. */
static void rcu_tasks_postscan(struct list_head *hop)
{
/*
* Exiting tasks may escape the tasklist scan. Those are vulnerable
* until their final schedule() with TASK_DEAD state. To cope with
* this, divide the fragile exit path part in two intersecting
* read side critical sections:
*
* 1) An _SRCU_ read side starting before calling exit_notify(),
* which may remove the task from the tasklist, and ending after
* the final preempt_disable() call in do_exit().
*
* 2) An _RCU_ read side starting with the final preempt_disable()
* call in do_exit() and ending with the final call to schedule()
* with TASK_DEAD state.
*
* This handles the part 1). And postgp will handle part 2) with a
* call to synchronize_rcu().
*/
synchronize_srcu(&tasks_rcu_exit_srcu);
}
/* See if tasks are still holding out, complain if so. */
static void check_holdout_task(struct task_struct *t,
bool needreport, bool *firstreport)
{
int cpu;
if (!READ_ONCE(t->rcu_tasks_holdout) ||
t->rcu_tasks_nvcsw != READ_ONCE(t->nvcsw) ||
!READ_ONCE(t->on_rq) ||
(IS_ENABLED(CONFIG_NO_HZ_FULL) &&
!is_idle_task(t) && t->rcu_tasks_idle_cpu >= 0)) {
WRITE_ONCE(t->rcu_tasks_holdout, false);
list_del_init(&t->rcu_tasks_holdout_list);
put_task_struct(t);
return;
}
rcu_request_urgent_qs_task(t);
if (!needreport)
return;
if (*firstreport) {
pr_err("INFO: rcu_tasks detected stalls on tasks:\n");
*firstreport = false;
}
cpu = task_cpu(t);
pr_alert("%p: %c%c nvcsw: %lu/%lu holdout: %d idle_cpu: %d/%d\n",
t, ".I"[is_idle_task(t)],
"N."[cpu < 0 || !tick_nohz_full_cpu(cpu)],
t->rcu_tasks_nvcsw, t->nvcsw, t->rcu_tasks_holdout,
t->rcu_tasks_idle_cpu, cpu);
sched_show_task(t);
}
/* Scan the holdout lists for tasks no longer holding out. */
static void check_all_holdout_tasks(struct list_head *hop,
bool needreport, bool *firstreport)
{
struct task_struct *t, *t1;
list_for_each_entry_safe(t, t1, hop, rcu_tasks_holdout_list) {
check_holdout_task(t, needreport, firstreport);
cond_resched();
}
}
/* Finish off the Tasks-RCU grace period. */
static void rcu_tasks_postgp(struct rcu_tasks *rtp)
{
/*
* Because ->on_rq and ->nvcsw are not guaranteed to have a full
* memory barriers prior to them in the schedule() path, memory
* reordering on other CPUs could cause their RCU-tasks read-side
* critical sections to extend past the end of the grace period.
* However, because these ->nvcsw updates are carried out with
* interrupts disabled, we can use synchronize_rcu() to force the
* needed ordering on all such CPUs.
*
* This synchronize_rcu() also confines all ->rcu_tasks_holdout
* accesses to be within the grace period, avoiding the need for
* memory barriers for ->rcu_tasks_holdout accesses.
*
* In addition, this synchronize_rcu() waits for exiting tasks
* to complete their final preempt_disable() region of execution,
* cleaning up after synchronize_srcu(&tasks_rcu_exit_srcu),
* enforcing the whole region before tasklist removal until
* the final schedule() with TASK_DEAD state to be an RCU TASKS
* read side critical section.
*/
synchronize_rcu();
}
void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func);
DEFINE_RCU_TASKS(rcu_tasks, rcu_tasks_wait_gp, call_rcu_tasks, "RCU Tasks");
/**
* call_rcu_tasks() - Queue an RCU for invocation task-based grace period
* @rhp: structure to be used for queueing the RCU updates.
* @func: actual callback function to be invoked after the grace period
*
* The callback function will be invoked some time after a full grace
* period elapses, in other words after all currently executing RCU
* read-side critical sections have completed. call_rcu_tasks() assumes
* that the read-side critical sections end at a voluntary context
* switch (not a preemption!), cond_resched_tasks_rcu_qs(), entry into idle,
* or transition to usermode execution. As such, there are no read-side
* primitives analogous to rcu_read_lock() and rcu_read_unlock() because
* this primitive is intended to determine that all tasks have passed
* through a safe state, not so much for data-structure synchronization.
*
* See the description of call_rcu() for more detailed information on
* memory ordering guarantees.
*/
void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func)
{
call_rcu_tasks_generic(rhp, func, &rcu_tasks);
}
EXPORT_SYMBOL_GPL(call_rcu_tasks);
/**
* synchronize_rcu_tasks - wait until an rcu-tasks grace period has elapsed.
*
* Control will return to the caller some time after a full rcu-tasks
* grace period has elapsed, in other words after all currently
* executing rcu-tasks read-side critical sections have elapsed. These
* read-side critical sections are delimited by calls to schedule(),
* cond_resched_tasks_rcu_qs(), idle execution, userspace execution, calls
* to synchronize_rcu_tasks(), and (in theory, anyway) cond_resched().
*
* This is a very specialized primitive, intended only for a few uses in
* tracing and other situations requiring manipulation of function
* preambles and profiling hooks. The synchronize_rcu_tasks() function
* is not (yet) intended for heavy use from multiple CPUs.
*
* See the description of synchronize_rcu() for more detailed information
* on memory ordering guarantees.
*/
void synchronize_rcu_tasks(void)
{
synchronize_rcu_tasks_generic(&rcu_tasks);
}
EXPORT_SYMBOL_GPL(synchronize_rcu_tasks);
/**
* rcu_barrier_tasks - Wait for in-flight call_rcu_tasks() callbacks.
*
* Although the current implementation is guaranteed to wait, it is not
* obligated to, for example, if there are no pending callbacks.
*/
void rcu_barrier_tasks(void)
{
rcu_barrier_tasks_generic(&rcu_tasks);
}
EXPORT_SYMBOL_GPL(rcu_barrier_tasks);
static int __init rcu_spawn_tasks_kthread(void)
{
cblist_init_generic(&rcu_tasks);
rcu_tasks.gp_sleep = HZ / 10;
rcu_tasks.init_fract = HZ / 10;
rcu_tasks.pregp_func = rcu_tasks_pregp_step;
rcu_tasks.pertask_func = rcu_tasks_pertask;
rcu_tasks.postscan_func = rcu_tasks_postscan;
rcu_tasks.holdouts_func = check_all_holdout_tasks;
rcu_tasks.postgp_func = rcu_tasks_postgp;
rcu_spawn_tasks_kthread_generic(&rcu_tasks);
return 0;
}
#if !defined(CONFIG_TINY_RCU)
void show_rcu_tasks_classic_gp_kthread(void)
{
show_rcu_tasks_generic_gp_kthread(&rcu_tasks, "");
}
EXPORT_SYMBOL_GPL(show_rcu_tasks_classic_gp_kthread);
#endif // !defined(CONFIG_TINY_RCU)
/*
* Contribute to protect against tasklist scan blind spot while the
* task is exiting and may be removed from the tasklist. See
* corresponding synchronize_srcu() for further details.
*/
void exit_tasks_rcu_start(void) __acquires(&tasks_rcu_exit_srcu)
{
current->rcu_tasks_idx = __srcu_read_lock(&tasks_rcu_exit_srcu);
}
/*
* Contribute to protect against tasklist scan blind spot while the
* task is exiting and may be removed from the tasklist. See
* corresponding synchronize_srcu() for further details.
*/
void exit_tasks_rcu_stop(void) __releases(&tasks_rcu_exit_srcu)
{
struct task_struct *t = current;
__srcu_read_unlock(&tasks_rcu_exit_srcu, t->rcu_tasks_idx);
}
/*
* Contribute to protect against tasklist scan blind spot while the
* task is exiting and may be removed from the tasklist. See
* corresponding synchronize_srcu() for further details.
*/
void exit_tasks_rcu_finish(void)
{
exit_tasks_rcu_stop();
exit_tasks_rcu_finish_trace(current);
}
#else /* #ifdef CONFIG_TASKS_RCU */
void exit_tasks_rcu_start(void) { }
void exit_tasks_rcu_stop(void) { }
void exit_tasks_rcu_finish(void) { exit_tasks_rcu_finish_trace(current); }
#endif /* #else #ifdef CONFIG_TASKS_RCU */
#ifdef CONFIG_TASKS_RUDE_RCU
////////////////////////////////////////////////////////////////////////
//
// "Rude" variant of Tasks RCU, inspired by Steve Rostedt's trick of
// passing an empty function to schedule_on_each_cpu(). This approach
// provides an asynchronous call_rcu_tasks_rude() API and batching of
// concurrent calls to the synchronous synchronize_rcu_tasks_rude() API.
// This invokes schedule_on_each_cpu() in order to send IPIs far and wide
// and induces otherwise unnecessary context switches on all online CPUs,
// whether idle or not.
//
// Callback handling is provided by the rcu_tasks_kthread() function.
//
// Ordering is provided by the scheduler's context-switch code.
// Empty function to allow workqueues to force a context switch.
static void rcu_tasks_be_rude(struct work_struct *work)
{
}
// Wait for one rude RCU-tasks grace period.
static void rcu_tasks_rude_wait_gp(struct rcu_tasks *rtp)
{
rtp->n_ipis += cpumask_weight(cpu_online_mask);
schedule_on_each_cpu(rcu_tasks_be_rude);
}
void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t func);
DEFINE_RCU_TASKS(rcu_tasks_rude, rcu_tasks_rude_wait_gp, call_rcu_tasks_rude,
"RCU Tasks Rude");
/**
* call_rcu_tasks_rude() - Queue a callback rude task-based grace period
* @rhp: structure to be used for queueing the RCU updates.
* @func: actual callback function to be invoked after the grace period
*
* The callback function will be invoked some time after a full grace
* period elapses, in other words after all currently executing RCU
* read-side critical sections have completed. call_rcu_tasks_rude()
* assumes that the read-side critical sections end at context switch,
* cond_resched_tasks_rcu_qs(), or transition to usermode execution (as
* usermode execution is schedulable). As such, there are no read-side
* primitives analogous to rcu_read_lock() and rcu_read_unlock() because
* this primitive is intended to determine that all tasks have passed
* through a safe state, not so much for data-structure synchronization.
*
* See the description of call_rcu() for more detailed information on
* memory ordering guarantees.
*/
void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t func)
{
call_rcu_tasks_generic(rhp, func, &rcu_tasks_rude);
}
EXPORT_SYMBOL_GPL(call_rcu_tasks_rude);
/**
* synchronize_rcu_tasks_rude - wait for a rude rcu-tasks grace period
*
* Control will return to the caller some time after a rude rcu-tasks
* grace period has elapsed, in other words after all currently
* executing rcu-tasks read-side critical sections have elapsed. These
* read-side critical sections are delimited by calls to schedule(),
* cond_resched_tasks_rcu_qs(), userspace execution (which is a schedulable
* context), and (in theory, anyway) cond_resched().
*
* This is a very specialized primitive, intended only for a few uses in
* tracing and other situations requiring manipulation of function preambles
* and profiling hooks. The synchronize_rcu_tasks_rude() function is not
* (yet) intended for heavy use from multiple CPUs.
*
* See the description of synchronize_rcu() for more detailed information
* on memory ordering guarantees.
*/
void synchronize_rcu_tasks_rude(void)
{
synchronize_rcu_tasks_generic(&rcu_tasks_rude);
}
EXPORT_SYMBOL_GPL(synchronize_rcu_tasks_rude);
/**
* rcu_barrier_tasks_rude - Wait for in-flight call_rcu_tasks_rude() callbacks.
*
* Although the current implementation is guaranteed to wait, it is not
* obligated to, for example, if there are no pending callbacks.
*/
void rcu_barrier_tasks_rude(void)
{
rcu_barrier_tasks_generic(&rcu_tasks_rude);
}
EXPORT_SYMBOL_GPL(rcu_barrier_tasks_rude);
static int __init rcu_spawn_tasks_rude_kthread(void)
{
cblist_init_generic(&rcu_tasks_rude);
rcu_tasks_rude.gp_sleep = HZ / 10;
rcu_spawn_tasks_kthread_generic(&rcu_tasks_rude);
return 0;
}
#if !defined(CONFIG_TINY_RCU)
void show_rcu_tasks_rude_gp_kthread(void)
{
show_rcu_tasks_generic_gp_kthread(&rcu_tasks_rude, "");
}
EXPORT_SYMBOL_GPL(show_rcu_tasks_rude_gp_kthread);
#endif // !defined(CONFIG_TINY_RCU)
#endif /* #ifdef CONFIG_TASKS_RUDE_RCU */
////////////////////////////////////////////////////////////////////////
//
// Tracing variant of Tasks RCU. This variant is designed to be used
// to protect tracing hooks, including those of BPF. This variant
// therefore:
//
// 1. Has explicit read-side markers to allow finite grace periods
// in the face of in-kernel loops for PREEMPT=n builds.
//
// 2. Protects code in the idle loop, exception entry/exit, and
// CPU-hotplug code paths, similar to the capabilities of SRCU.
//
// 3. Avoids expensive read-side instructions, having overhead similar
// to that of Preemptible RCU.
//
// There are of course downsides. For example, the grace-period code
// can send IPIs to CPUs, even when those CPUs are in the idle loop or
// in nohz_full userspace. If needed, these downsides can be at least
// partially remedied.
//
// Perhaps most important, this variant of RCU does not affect the vanilla
// flavors, rcu_preempt and rcu_sched. The fact that RCU Tasks Trace
// readers can operate from idle, offline, and exception entry/exit in no
// way allows rcu_preempt and rcu_sched readers to also do so.
//
// The implementation uses rcu_tasks_wait_gp(), which relies on function
// pointers in the rcu_tasks structure. The rcu_spawn_tasks_trace_kthread()
// function sets these function pointers up so that rcu_tasks_wait_gp()
// invokes these functions in this order:
//
// rcu_tasks_trace_pregp_step():
// Disables CPU hotplug, adds all currently executing tasks to the
// holdout list, then checks the state of all tasks that blocked
// or were preempted within their current RCU Tasks Trace read-side
// critical section, adding them to the holdout list if appropriate.
// Finally, this function re-enables CPU hotplug.
// The ->pertask_func() pointer is NULL, so there is no per-task processing.
// rcu_tasks_trace_postscan():
// Invokes synchronize_rcu() to wait for late-stage exiting tasks
// to finish exiting.
// check_all_holdout_tasks_trace(), repeatedly until holdout list is empty:
// Scans the holdout list, attempting to identify a quiescent state
// for each task on the list. If there is a quiescent state, the
// corresponding task is removed from the holdout list. Once this
// list is empty, the grace period has completed.
// rcu_tasks_trace_postgp():
// Provides the needed full memory barrier and does debug checks.
//
// The exit_tasks_rcu_finish_trace() synchronizes with exiting tasks.
//
// Pre-grace-period update-side code is ordered before the grace period
// via the ->cbs_lock and barriers in rcu_tasks_kthread(). Pre-grace-period
// read-side code is ordered before the grace period by atomic operations
// on .b.need_qs flag of each task involved in this process, or by scheduler
// context-switch ordering (for locked-down non-running readers).
// The lockdep state must be outside of #ifdef to be useful.
#ifdef CONFIG_DEBUG_LOCK_ALLOC
static struct lock_class_key rcu_lock_trace_key;
struct lockdep_map rcu_trace_lock_map =
STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_trace", &rcu_lock_trace_key);
EXPORT_SYMBOL_GPL(rcu_trace_lock_map);
#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
#ifdef CONFIG_TASKS_TRACE_RCU
// Record outstanding IPIs to each CPU. No point in sending two...
static DEFINE_PER_CPU(bool, trc_ipi_to_cpu);
// The number of detections of task quiescent state relying on
// heavyweight readers executing explicit memory barriers.
static unsigned long n_heavy_reader_attempts;
static unsigned long n_heavy_reader_updates;
static unsigned long n_heavy_reader_ofl_updates;
static unsigned long n_trc_holdouts;
void call_rcu_tasks_trace(struct rcu_head *rhp, rcu_callback_t func);
DEFINE_RCU_TASKS(rcu_tasks_trace, rcu_tasks_wait_gp, call_rcu_tasks_trace,
"RCU Tasks Trace");
/* Load from ->trc_reader_special.b.need_qs with proper ordering. */
static u8 rcu_ld_need_qs(struct task_struct *t)
{
smp_mb(); // Enforce full grace-period ordering.
return smp_load_acquire(&t->trc_reader_special.b.need_qs);
}
/* Store to ->trc_reader_special.b.need_qs with proper ordering. */
static void rcu_st_need_qs(struct task_struct *t, u8 v)
{
smp_store_release(&t->trc_reader_special.b.need_qs, v);
smp_mb(); // Enforce full grace-period ordering.
}
/*
* Do a cmpxchg() on ->trc_reader_special.b.need_qs, allowing for
* the four-byte operand-size restriction of some platforms.
* Returns the old value, which is often ignored.
*/
u8 rcu_trc_cmpxchg_need_qs(struct task_struct *t, u8 old, u8 new)
{
union rcu_special ret;
union rcu_special trs_old = READ_ONCE(t->trc_reader_special);
union rcu_special trs_new = trs_old;
if (trs_old.b.need_qs != old)
return trs_old.b.need_qs;
trs_new.b.need_qs = new;
ret.s = cmpxchg(&t->trc_reader_special.s, trs_old.s, trs_new.s);
return ret.b.need_qs;
}
EXPORT_SYMBOL_GPL(rcu_trc_cmpxchg_need_qs);
/*
* If we are the last reader, signal the grace-period kthread.
* Also remove from the per-CPU list of blocked tasks.
*/
void rcu_read_unlock_trace_special(struct task_struct *t)
{
unsigned long flags;
struct rcu_tasks_percpu *rtpcp;
union rcu_special trs;
// Open-coded full-word version of rcu_ld_need_qs().
smp_mb(); // Enforce full grace-period ordering.
trs = smp_load_acquire(&t->trc_reader_special);
if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB) && t->trc_reader_special.b.need_mb)
smp_mb(); // Pairs with update-side barriers.
// Update .need_qs before ->trc_reader_nesting for irq/NMI handlers.
if (trs.b.need_qs == (TRC_NEED_QS_CHECKED | TRC_NEED_QS)) {
u8 result = rcu_trc_cmpxchg_need_qs(t, TRC_NEED_QS_CHECKED | TRC_NEED_QS,
TRC_NEED_QS_CHECKED);
WARN_ONCE(result != trs.b.need_qs, "%s: result = %d", __func__, result);
}
if (trs.b.blocked) {
rtpcp = per_cpu_ptr(rcu_tasks_trace.rtpcpu, t->trc_blkd_cpu);
raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
list_del_init(&t->trc_blkd_node);
WRITE_ONCE(t->trc_reader_special.b.blocked, false);
raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
}
WRITE_ONCE(t->trc_reader_nesting, 0);
}
EXPORT_SYMBOL_GPL(rcu_read_unlock_trace_special);
/* Add a newly blocked reader task to its CPU's list. */
void rcu_tasks_trace_qs_blkd(struct task_struct *t)
{
unsigned long flags;
struct rcu_tasks_percpu *rtpcp;
local_irq_save(flags);
rtpcp = this_cpu_ptr(rcu_tasks_trace.rtpcpu);
raw_spin_lock_rcu_node(rtpcp); // irqs already disabled
t->trc_blkd_cpu = smp_processor_id();
if (!rtpcp->rtp_blkd_tasks.next)
INIT_LIST_HEAD(&rtpcp->rtp_blkd_tasks);
list_add(&t->trc_blkd_node, &rtpcp->rtp_blkd_tasks);
WRITE_ONCE(t->trc_reader_special.b.blocked, true);
raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
}
EXPORT_SYMBOL_GPL(rcu_tasks_trace_qs_blkd);
/* Add a task to the holdout list, if it is not already on the list. */
static void trc_add_holdout(struct task_struct *t, struct list_head *bhp)
{
if (list_empty(&t->trc_holdout_list)) {
get_task_struct(t);
list_add(&t->trc_holdout_list, bhp);
n_trc_holdouts++;
}
}
/* Remove a task from the holdout list, if it is in fact present. */
static void trc_del_holdout(struct task_struct *t)
{
if (!list_empty(&t->trc_holdout_list)) {
list_del_init(&t->trc_holdout_list);
put_task_struct(t);
n_trc_holdouts--;
}
}
/* IPI handler to check task state. */
static void trc_read_check_handler(void *t_in)
{
int nesting;
struct task_struct *t = current;
struct task_struct *texp = t_in;
// If the task is no longer running on this CPU, leave.
if (unlikely(texp != t))
goto reset_ipi; // Already on holdout list, so will check later.
// If the task is not in a read-side critical section, and
// if this is the last reader, awaken the grace-period kthread.
nesting = READ_ONCE(t->trc_reader_nesting);
if (likely(!nesting)) {
rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS_CHECKED);
goto reset_ipi;
}
// If we are racing with an rcu_read_unlock_trace(), try again later.
if (unlikely(nesting < 0))
goto reset_ipi;
// Get here if the task is in a read-side critical section.
// Set its state so that it will update state for the grace-period
// kthread upon exit from that critical section.
rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS | TRC_NEED_QS_CHECKED);
reset_ipi:
// Allow future IPIs to be sent on CPU and for task.
// Also order this IPI handler against any later manipulations of
// the intended task.
smp_store_release(per_cpu_ptr(&trc_ipi_to_cpu, smp_processor_id()), false); // ^^^
smp_store_release(&texp->trc_ipi_to_cpu, -1); // ^^^
}
/* Callback function for scheduler to check locked-down task. */
static int trc_inspect_reader(struct task_struct *t, void *bhp_in)
{
struct list_head *bhp = bhp_in;
int cpu = task_cpu(t);
int nesting;
bool ofl = cpu_is_offline(cpu);
if (task_curr(t) && !ofl) {
// If no chance of heavyweight readers, do it the hard way.
if (!IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
return -EINVAL;
// If heavyweight readers are enabled on the remote task,
// we can inspect its state despite its currently running.
// However, we cannot safely change its state.
n_heavy_reader_attempts++;
// Check for "running" idle tasks on offline CPUs.
if (!rcu_dynticks_zero_in_eqs(cpu, &t->trc_reader_nesting))
return -EINVAL; // No quiescent state, do it the hard way.
n_heavy_reader_updates++;
nesting = 0;
} else {
// The task is not running, so C-language access is safe.
nesting = t->trc_reader_nesting;
WARN_ON_ONCE(ofl && task_curr(t) && !is_idle_task(t));
if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB) && ofl)
n_heavy_reader_ofl_updates++;
}
// If not exiting a read-side critical section, mark as checked
// so that the grace-period kthread will remove it from the
// holdout list.
if (!nesting) {
rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS_CHECKED);
return 0; // In QS, so done.
}
if (nesting < 0)
return -EINVAL; // Reader transitioning, try again later.
// The task is in a read-side critical section, so set up its
// state so that it will update state upon exit from that critical
// section.
if (!rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS | TRC_NEED_QS_CHECKED))
trc_add_holdout(t, bhp);
return 0;
}
/* Attempt to extract the state for the specified task. */
static void trc_wait_for_one_reader(struct task_struct *t,
struct list_head *bhp)
{
int cpu;
// If a previous IPI is still in flight, let it complete.
if (smp_load_acquire(&t->trc_ipi_to_cpu) != -1) // Order IPI
return;
// The current task had better be in a quiescent state.
if (t == current) {
rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS_CHECKED);
WARN_ON_ONCE(READ_ONCE(t->trc_reader_nesting));
return;
}
// Attempt to nail down the task for inspection.
get_task_struct(t);
if (!task_call_func(t, trc_inspect_reader, bhp)) {
put_task_struct(t);
return;
}
put_task_struct(t);
// If this task is not yet on the holdout list, then we are in
// an RCU read-side critical section. Otherwise, the invocation of
// trc_add_holdout() that added it to the list did the necessary
// get_task_struct(). Either way, the task cannot be freed out
// from under this code.
// If currently running, send an IPI, either way, add to list.
trc_add_holdout(t, bhp);
if (task_curr(t) &&
time_after(jiffies + 1, rcu_tasks_trace.gp_start + rcu_task_ipi_delay)) {
// The task is currently running, so try IPIing it.
cpu = task_cpu(t);
// If there is already an IPI outstanding, let it happen.
if (per_cpu(trc_ipi_to_cpu, cpu) || t->trc_ipi_to_cpu >= 0)
return;
per_cpu(trc_ipi_to_cpu, cpu) = true;
t->trc_ipi_to_cpu = cpu;
rcu_tasks_trace.n_ipis++;
if (smp_call_function_single(cpu, trc_read_check_handler, t, 0)) {
// Just in case there is some other reason for
// failure than the target CPU being offline.
WARN_ONCE(1, "%s(): smp_call_function_single() failed for CPU: %d\n",
__func__, cpu);
rcu_tasks_trace.n_ipis_fails++;
per_cpu(trc_ipi_to_cpu, cpu) = false;
t->trc_ipi_to_cpu = -1;
}
}
}
/*
* Initialize for first-round processing for the specified task.
* Return false if task is NULL or already taken care of, true otherwise.
*/
static bool rcu_tasks_trace_pertask_prep(struct task_struct *t, bool notself)
{
// During early boot when there is only the one boot CPU, there
// is no idle task for the other CPUs. Also, the grace-period
// kthread is always in a quiescent state. In addition, just return
// if this task is already on the list.
if (unlikely(t == NULL) || (t == current && notself) || !list_empty(&t->trc_holdout_list))
return false;
rcu_st_need_qs(t, 0);
t->trc_ipi_to_cpu = -1;
return true;
}
/* Do first-round processing for the specified task. */
static void rcu_tasks_trace_pertask(struct task_struct *t, struct list_head *hop)
{
if (rcu_tasks_trace_pertask_prep(t, true))
trc_wait_for_one_reader(t, hop);
}
/* Initialize for a new RCU-tasks-trace grace period. */
static void rcu_tasks_trace_pregp_step(struct list_head *hop)
{
LIST_HEAD(blkd_tasks);
int cpu;
unsigned long flags;
struct rcu_tasks_percpu *rtpcp;
struct task_struct *t;
// There shouldn't be any old IPIs, but...
for_each_possible_cpu(cpu)
WARN_ON_ONCE(per_cpu(trc_ipi_to_cpu, cpu));
// Disable CPU hotplug across the CPU scan for the benefit of
// any IPIs that might be needed. This also waits for all readers
// in CPU-hotplug code paths.
cpus_read_lock();
// These rcu_tasks_trace_pertask_prep() calls are serialized to
// allow safe access to the hop list.
for_each_online_cpu(cpu) {
rcu_read_lock();
t = cpu_curr_snapshot(cpu);
if (rcu_tasks_trace_pertask_prep(t, true))
trc_add_holdout(t, hop);
rcu_read_unlock();
cond_resched_tasks_rcu_qs();
}
// Only after all running tasks have been accounted for is it
// safe to take care of the tasks that have blocked within their
// current RCU tasks trace read-side critical section.
for_each_possible_cpu(cpu) {
rtpcp = per_cpu_ptr(rcu_tasks_trace.rtpcpu, cpu);
raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
list_splice_init(&rtpcp->rtp_blkd_tasks, &blkd_tasks);
while (!list_empty(&blkd_tasks)) {
rcu_read_lock();
t = list_first_entry(&blkd_tasks, struct task_struct, trc_blkd_node);
list_del_init(&t->trc_blkd_node);
list_add(&t->trc_blkd_node, &rtpcp->rtp_blkd_tasks);
raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
rcu_tasks_trace_pertask(t, hop);
rcu_read_unlock();
raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
}
raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
cond_resched_tasks_rcu_qs();
}
// Re-enable CPU hotplug now that the holdout list is populated.
cpus_read_unlock();
}
/*
* Do intermediate processing between task and holdout scans.
*/
static void rcu_tasks_trace_postscan(struct list_head *hop)
{
// Wait for late-stage exiting tasks to finish exiting.
// These might have passed the call to exit_tasks_rcu_finish().
// If you remove the following line, update rcu_trace_implies_rcu_gp()!!!
synchronize_rcu();
// Any tasks that exit after this point will set
// TRC_NEED_QS_CHECKED in ->trc_reader_special.b.need_qs.
}
/* Communicate task state back to the RCU tasks trace stall warning request. */
struct trc_stall_chk_rdr {
int nesting;
int ipi_to_cpu;
u8 needqs;
};
static int trc_check_slow_task(struct task_struct *t, void *arg)
{
struct trc_stall_chk_rdr *trc_rdrp = arg;
if (task_curr(t) && cpu_online(task_cpu(t)))
return false; // It is running, so decline to inspect it.
trc_rdrp->nesting = READ_ONCE(t->trc_reader_nesting);
trc_rdrp->ipi_to_cpu = READ_ONCE(t->trc_ipi_to_cpu);
trc_rdrp->needqs = rcu_ld_need_qs(t);
return true;
}
/* Show the state of a task stalling the current RCU tasks trace GP. */
static void show_stalled_task_trace(struct task_struct *t, bool *firstreport)
{
int cpu;
struct trc_stall_chk_rdr trc_rdr;
bool is_idle_tsk = is_idle_task(t);
if (*firstreport) {
pr_err("INFO: rcu_tasks_trace detected stalls on tasks:\n");
*firstreport = false;
}
cpu = task_cpu(t);
if (!task_call_func(t, trc_check_slow_task, &trc_rdr))
pr_alert("P%d: %c%c\n",
t->pid,
".I"[t->trc_ipi_to_cpu >= 0],
".i"[is_idle_tsk]);
else
pr_alert("P%d: %c%c%c%c nesting: %d%c%c cpu: %d%s\n",
t->pid,
".I"[trc_rdr.ipi_to_cpu >= 0],
".i"[is_idle_tsk],
".N"[cpu >= 0 && tick_nohz_full_cpu(cpu)],
".B"[!!data_race(t->trc_reader_special.b.blocked)],
trc_rdr.nesting,
" !CN"[trc_rdr.needqs & 0x3],
" ?"[trc_rdr.needqs > 0x3],
cpu, cpu_online(cpu) ? "" : "(offline)");
sched_show_task(t);
}
/* List stalled IPIs for RCU tasks trace. */
static void show_stalled_ipi_trace(void)
{
int cpu;
for_each_possible_cpu(cpu)
if (per_cpu(trc_ipi_to_cpu, cpu))
pr_alert("\tIPI outstanding to CPU %d\n", cpu);
}
/* Do one scan of the holdout list. */
static void check_all_holdout_tasks_trace(struct list_head *hop,
bool needreport, bool *firstreport)
{
struct task_struct *g, *t;
// Disable CPU hotplug across the holdout list scan for IPIs.
cpus_read_lock();
list_for_each_entry_safe(t, g, hop, trc_holdout_list) {
// If safe and needed, try to check the current task.
if (READ_ONCE(t->trc_ipi_to_cpu) == -1 &&
!(rcu_ld_need_qs(t) & TRC_NEED_QS_CHECKED))
trc_wait_for_one_reader(t, hop);
// If check succeeded, remove this task from the list.
if (smp_load_acquire(&t->trc_ipi_to_cpu) == -1 &&
rcu_ld_need_qs(t) == TRC_NEED_QS_CHECKED)
trc_del_holdout(t);
else if (needreport)
show_stalled_task_trace(t, firstreport);
cond_resched_tasks_rcu_qs();
}
// Re-enable CPU hotplug now that the holdout list scan has completed.
cpus_read_unlock();
if (needreport) {
if (*firstreport)
pr_err("INFO: rcu_tasks_trace detected stalls? (Late IPI?)\n");
show_stalled_ipi_trace();
}
}
static void rcu_tasks_trace_empty_fn(void *unused)
{
}
/* Wait for grace period to complete and provide ordering. */
static void rcu_tasks_trace_postgp(struct rcu_tasks *rtp)
{
int cpu;
// Wait for any lingering IPI handlers to complete. Note that
// if a CPU has gone offline or transitioned to userspace in the
// meantime, all IPI handlers should have been drained beforehand.
// Yes, this assumes that CPUs process IPIs in order. If that ever
// changes, there will need to be a recheck and/or timed wait.
for_each_online_cpu(cpu)
if (WARN_ON_ONCE(smp_load_acquire(per_cpu_ptr(&trc_ipi_to_cpu, cpu))))
smp_call_function_single(cpu, rcu_tasks_trace_empty_fn, NULL, 1);
smp_mb(); // Caller's code must be ordered after wakeup.
// Pairs with pretty much every ordering primitive.
}
/* Report any needed quiescent state for this exiting task. */
static void exit_tasks_rcu_finish_trace(struct task_struct *t)
{
union rcu_special trs = READ_ONCE(t->trc_reader_special);
rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS_CHECKED);
WARN_ON_ONCE(READ_ONCE(t->trc_reader_nesting));
if (WARN_ON_ONCE(rcu_ld_need_qs(t) & TRC_NEED_QS || trs.b.blocked))
rcu_read_unlock_trace_special(t);
else
WRITE_ONCE(t->trc_reader_nesting, 0);
}
/**
* call_rcu_tasks_trace() - Queue a callback trace task-based grace period
* @rhp: structure to be used for queueing the RCU updates.
* @func: actual callback function to be invoked after the grace period
*
* The callback function will be invoked some time after a trace rcu-tasks
* grace period elapses, in other words after all currently executing
* trace rcu-tasks read-side critical sections have completed. These
* read-side critical sections are delimited by calls to rcu_read_lock_trace()
* and rcu_read_unlock_trace().
*
* See the description of call_rcu() for more detailed information on
* memory ordering guarantees.
*/
void call_rcu_tasks_trace(struct rcu_head *rhp, rcu_callback_t func)
{
call_rcu_tasks_generic(rhp, func, &rcu_tasks_trace);
}
EXPORT_SYMBOL_GPL(call_rcu_tasks_trace);
/**
* synchronize_rcu_tasks_trace - wait for a trace rcu-tasks grace period
*
* Control will return to the caller some time after a trace rcu-tasks
* grace period has elapsed, in other words after all currently executing
* trace rcu-tasks read-side critical sections have elapsed. These read-side
* critical sections are delimited by calls to rcu_read_lock_trace()
* and rcu_read_unlock_trace().
*
* This is a very specialized primitive, intended only for a few uses in
* tracing and other situations requiring manipulation of function preambles
* and profiling hooks. The synchronize_rcu_tasks_trace() function is not
* (yet) intended for heavy use from multiple CPUs.
*
* See the description of synchronize_rcu() for more detailed information
* on memory ordering guarantees.
*/
void synchronize_rcu_tasks_trace(void)
{
RCU_LOCKDEP_WARN(lock_is_held(&rcu_trace_lock_map), "Illegal synchronize_rcu_tasks_trace() in RCU Tasks Trace read-side critical section");
synchronize_rcu_tasks_generic(&rcu_tasks_trace);
}
EXPORT_SYMBOL_GPL(synchronize_rcu_tasks_trace);
/**
* rcu_barrier_tasks_trace - Wait for in-flight call_rcu_tasks_trace() callbacks.
*
* Although the current implementation is guaranteed to wait, it is not
* obligated to, for example, if there are no pending callbacks.
*/
void rcu_barrier_tasks_trace(void)
{
rcu_barrier_tasks_generic(&rcu_tasks_trace);
}
EXPORT_SYMBOL_GPL(rcu_barrier_tasks_trace);
static int __init rcu_spawn_tasks_trace_kthread(void)
{
cblist_init_generic(&rcu_tasks_trace);
if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB)) {
rcu_tasks_trace.gp_sleep = HZ / 10;
rcu_tasks_trace.init_fract = HZ / 10;
} else {
rcu_tasks_trace.gp_sleep = HZ / 200;
if (rcu_tasks_trace.gp_sleep <= 0)
rcu_tasks_trace.gp_sleep = 1;
rcu_tasks_trace.init_fract = HZ / 200;
if (rcu_tasks_trace.init_fract <= 0)
rcu_tasks_trace.init_fract = 1;
}
rcu_tasks_trace.pregp_func = rcu_tasks_trace_pregp_step;
rcu_tasks_trace.postscan_func = rcu_tasks_trace_postscan;
rcu_tasks_trace.holdouts_func = check_all_holdout_tasks_trace;
rcu_tasks_trace.postgp_func = rcu_tasks_trace_postgp;
rcu_spawn_tasks_kthread_generic(&rcu_tasks_trace);
return 0;
}
#if !defined(CONFIG_TINY_RCU)
void show_rcu_tasks_trace_gp_kthread(void)
{
char buf[64];
sprintf(buf, "N%lu h:%lu/%lu/%lu",
data_race(n_trc_holdouts),
data_race(n_heavy_reader_ofl_updates),
data_race(n_heavy_reader_updates),
data_race(n_heavy_reader_attempts));
show_rcu_tasks_generic_gp_kthread(&rcu_tasks_trace, buf);
}
EXPORT_SYMBOL_GPL(show_rcu_tasks_trace_gp_kthread);
#endif // !defined(CONFIG_TINY_RCU)
#else /* #ifdef CONFIG_TASKS_TRACE_RCU */
static void exit_tasks_rcu_finish_trace(struct task_struct *t) { }
#endif /* #else #ifdef CONFIG_TASKS_TRACE_RCU */
#ifndef CONFIG_TINY_RCU
void show_rcu_tasks_gp_kthreads(void)
{
show_rcu_tasks_classic_gp_kthread();
show_rcu_tasks_rude_gp_kthread();
show_rcu_tasks_trace_gp_kthread();
}
#endif /* #ifndef CONFIG_TINY_RCU */
#ifdef CONFIG_PROVE_RCU
struct rcu_tasks_test_desc {
struct rcu_head rh;
const char *name;
bool notrun;
unsigned long runstart;
};
static struct rcu_tasks_test_desc tests[] = {
{
.name = "call_rcu_tasks()",
/* If not defined, the test is skipped. */
.notrun = IS_ENABLED(CONFIG_TASKS_RCU),
},
{
.name = "call_rcu_tasks_rude()",
/* If not defined, the test is skipped. */
.notrun = IS_ENABLED(CONFIG_TASKS_RUDE_RCU),
},
{
.name = "call_rcu_tasks_trace()",
/* If not defined, the test is skipped. */
.notrun = IS_ENABLED(CONFIG_TASKS_TRACE_RCU)
}
};
static void test_rcu_tasks_callback(struct rcu_head *rhp)
{
struct rcu_tasks_test_desc *rttd =
container_of(rhp, struct rcu_tasks_test_desc, rh);
pr_info("Callback from %s invoked.\n", rttd->name);
rttd->notrun = false;
}
static void rcu_tasks_initiate_self_tests(void)
{
pr_info("Running RCU-tasks wait API self tests\n");
#ifdef CONFIG_TASKS_RCU
tests[0].runstart = jiffies;
synchronize_rcu_tasks();
call_rcu_tasks(&tests[0].rh, test_rcu_tasks_callback);
#endif
#ifdef CONFIG_TASKS_RUDE_RCU
tests[1].runstart = jiffies;
synchronize_rcu_tasks_rude();
call_rcu_tasks_rude(&tests[1].rh, test_rcu_tasks_callback);
#endif
#ifdef CONFIG_TASKS_TRACE_RCU
tests[2].runstart = jiffies;
synchronize_rcu_tasks_trace();
call_rcu_tasks_trace(&tests[2].rh, test_rcu_tasks_callback);
#endif
}
/*
* Return: 0 - test passed
* 1 - test failed, but have not timed out yet
* -1 - test failed and timed out
*/
static int rcu_tasks_verify_self_tests(void)
{
int ret = 0;
int i;
unsigned long bst = rcu_task_stall_timeout;
if (bst <= 0 || bst > RCU_TASK_BOOT_STALL_TIMEOUT)
bst = RCU_TASK_BOOT_STALL_TIMEOUT;
for (i = 0; i < ARRAY_SIZE(tests); i++) {
while (tests[i].notrun) { // still hanging.
if (time_after(jiffies, tests[i].runstart + bst)) {
pr_err("%s has failed boot-time tests.\n", tests[i].name);
ret = -1;
break;
}
ret = 1;
break;
}
}
WARN_ON(ret < 0);
return ret;
}
/*
* Repeat the rcu_tasks_verify_self_tests() call once every second until the
* test passes or has timed out.
*/
static struct delayed_work rcu_tasks_verify_work;
static void rcu_tasks_verify_work_fn(struct work_struct *work __maybe_unused)
{
int ret = rcu_tasks_verify_self_tests();
if (ret <= 0)
return;
/* Test fails but not timed out yet, reschedule another check */
schedule_delayed_work(&rcu_tasks_verify_work, HZ);
}
static int rcu_tasks_verify_schedule_work(void)
{
INIT_DELAYED_WORK(&rcu_tasks_verify_work, rcu_tasks_verify_work_fn);
rcu_tasks_verify_work_fn(NULL);
return 0;
}
late_initcall(rcu_tasks_verify_schedule_work);
#else /* #ifdef CONFIG_PROVE_RCU */
static void rcu_tasks_initiate_self_tests(void) { }
#endif /* #else #ifdef CONFIG_PROVE_RCU */
void __init rcu_init_tasks_generic(void)
{
#ifdef CONFIG_TASKS_RCU
rcu_spawn_tasks_kthread();
#endif
#ifdef CONFIG_TASKS_RUDE_RCU
rcu_spawn_tasks_rude_kthread();
#endif
#ifdef CONFIG_TASKS_TRACE_RCU
rcu_spawn_tasks_trace_kthread();
#endif
// Run the self-tests.
rcu_tasks_initiate_self_tests();
}
#else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
static inline void rcu_tasks_bootup_oddness(void) {}
#endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */
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