linux/kernel/rcu/tree_plugin.h
Linus Torvalds e5e726f7bb Updates for locking and atomics:
The regular pile:
 
   - A few improvements to the mutex code
 
   - Documentation updates for atomics to clarify the difference between
     cmpxchg() and try_cmpxchg() and to explain the forward progress
     expectations.
 
   - Simplification of the atomics fallback generator
 
   - The addition of arch_atomic_long*() variants and generic arch_*()
     bitops based on them.
 
   - Add the missing might_sleep() invocations to the down*() operations of
     semaphores.
 
 The PREEMPT_RT locking core:
 
   - Scheduler updates to support the state preserving mechanism for
     'sleeping' spin- and rwlocks on RT. This mechanism is carefully
     preserving the state of the task when blocking on a 'sleeping' spin- or
     rwlock and takes regular wake-ups targeted at the same task into
     account. The preserved or updated (via a regular wakeup) state is
     restored when the lock has been acquired.
 
   - Restructuring of the rtmutex code so it can be utilized and extended
     for the RT specific lock variants.
 
   - Restructuring of the ww_mutex code to allow sharing of the ww_mutex
     specific functionality for rtmutex based ww_mutexes.
 
   - Header file disentangling to allow substitution of the regular lock
     implementations with the PREEMPT_RT variants without creating an
     unmaintainable #ifdef mess.
 
   - Shared base code for the PREEMPT_RT specific rw_semaphore and rwlock
     implementations. Contrary to the regular rw_semaphores and rwlocks the
     PREEMPT_RT implementation is writer unfair because it is infeasible to
     do priority inheritance on multiple readers. Experience over the years
     has shown that real-time workloads are not the typical workloads which
     are sensitive to writer starvation. The alternative solution would be
     to allow only a single reader which has been tried and discarded as it
     is a major bottleneck especially for mmap_sem. Aside of that many of
     the writer starvation critical usage sites have been converted to a
     writer side mutex/spinlock and RCU read side protections in the past
     decade so that the issue is less prominent than it used to be.
 
   - The actual rtmutex based lock substitutions for PREEMPT_RT enabled
     kernels which affect mutex, ww_mutex, rw_semaphore, spinlock_t and
     rwlock_t. The spin/rw_lock*() functions disable migration across the
     critical section to preserve the existing semantics vs. per CPU
     variables.
 
   - Rework of the futex REQUEUE_PI mechanism to handle the case of early
     wake-ups which interleave with a re-queue operation to prevent the
     situation that a task would be blocked on both the rtmutex associated
     to the outer futex and the rtmutex based hash bucket spinlock.
 
     While this situation cannot happen on !RT enabled kernels the changes
     make the underlying concurrency problems easier to understand in
     general. As a result the difference between !RT and RT kernels is
     reduced to the handling of waiting for the critical section. !RT
     kernels simply spin-wait as before and RT kernels utilize rcu_wait().
 
   - The substitution of local_lock for PREEMPT_RT with a spinlock which
     protects the critical section while staying preemptible. The CPU
     locality is established by disabling migration.
 
   The underlying concepts of this code have been in use in PREEMPT_RT for
   way more than a decade. The code has been refactored several times over
   the years and this final incarnation has been optimized once again to be
   as non-intrusive as possible, i.e. the RT specific parts are mostly
   isolated.
 
   It has been extensively tested in the 5.14-rt patch series and it has
   been verified that !RT kernels are not affected by these changes.
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Merge tag 'locking-core-2021-08-30' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull locking and atomics updates from Thomas Gleixner:
 "The regular pile:

   - A few improvements to the mutex code

   - Documentation updates for atomics to clarify the difference between
     cmpxchg() and try_cmpxchg() and to explain the forward progress
     expectations.

   - Simplification of the atomics fallback generator

   - The addition of arch_atomic_long*() variants and generic arch_*()
     bitops based on them.

   - Add the missing might_sleep() invocations to the down*() operations
     of semaphores.

  The PREEMPT_RT locking core:

   - Scheduler updates to support the state preserving mechanism for
     'sleeping' spin- and rwlocks on RT.

     This mechanism is carefully preserving the state of the task when
     blocking on a 'sleeping' spin- or rwlock and takes regular wake-ups
     targeted at the same task into account. The preserved or updated
     (via a regular wakeup) state is restored when the lock has been
     acquired.

   - Restructuring of the rtmutex code so it can be utilized and
     extended for the RT specific lock variants.

   - Restructuring of the ww_mutex code to allow sharing of the ww_mutex
     specific functionality for rtmutex based ww_mutexes.

   - Header file disentangling to allow substitution of the regular lock
     implementations with the PREEMPT_RT variants without creating an
     unmaintainable #ifdef mess.

   - Shared base code for the PREEMPT_RT specific rw_semaphore and
     rwlock implementations.

     Contrary to the regular rw_semaphores and rwlocks the PREEMPT_RT
     implementation is writer unfair because it is infeasible to do
     priority inheritance on multiple readers. Experience over the years
     has shown that real-time workloads are not the typical workloads
     which are sensitive to writer starvation.

     The alternative solution would be to allow only a single reader
     which has been tried and discarded as it is a major bottleneck
     especially for mmap_sem. Aside of that many of the writer
     starvation critical usage sites have been converted to a writer
     side mutex/spinlock and RCU read side protections in the past
     decade so that the issue is less prominent than it used to be.

   - The actual rtmutex based lock substitutions for PREEMPT_RT enabled
     kernels which affect mutex, ww_mutex, rw_semaphore, spinlock_t and
     rwlock_t. The spin/rw_lock*() functions disable migration across
     the critical section to preserve the existing semantics vs per-CPU
     variables.

   - Rework of the futex REQUEUE_PI mechanism to handle the case of
     early wake-ups which interleave with a re-queue operation to
     prevent the situation that a task would be blocked on both the
     rtmutex associated to the outer futex and the rtmutex based hash
     bucket spinlock.

     While this situation cannot happen on !RT enabled kernels the
     changes make the underlying concurrency problems easier to
     understand in general. As a result the difference between !RT and
     RT kernels is reduced to the handling of waiting for the critical
     section. !RT kernels simply spin-wait as before and RT kernels
     utilize rcu_wait().

   - The substitution of local_lock for PREEMPT_RT with a spinlock which
     protects the critical section while staying preemptible. The CPU
     locality is established by disabling migration.

  The underlying concepts of this code have been in use in PREEMPT_RT for
  way more than a decade. The code has been refactored several times over
  the years and this final incarnation has been optimized once again to be
  as non-intrusive as possible, i.e. the RT specific parts are mostly
  isolated.

  It has been extensively tested in the 5.14-rt patch series and it has
  been verified that !RT kernels are not affected by these changes"

* tag 'locking-core-2021-08-30' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (92 commits)
  locking/rtmutex: Return success on deadlock for ww_mutex waiters
  locking/rtmutex: Prevent spurious EDEADLK return caused by ww_mutexes
  locking/rtmutex: Dequeue waiter on ww_mutex deadlock
  locking/rtmutex: Dont dereference waiter lockless
  locking/semaphore: Add might_sleep() to down_*() family
  locking/ww_mutex: Initialize waiter.ww_ctx properly
  static_call: Update API documentation
  locking/local_lock: Add PREEMPT_RT support
  locking/spinlock/rt: Prepare for RT local_lock
  locking/rtmutex: Add adaptive spinwait mechanism
  locking/rtmutex: Implement equal priority lock stealing
  preempt: Adjust PREEMPT_LOCK_OFFSET for RT
  locking/rtmutex: Prevent lockdep false positive with PI futexes
  futex: Prevent requeue_pi() lock nesting issue on RT
  futex: Simplify handle_early_requeue_pi_wakeup()
  futex: Reorder sanity checks in futex_requeue()
  futex: Clarify comment in futex_requeue()
  futex: Restructure futex_requeue()
  futex: Correct the number of requeued waiters for PI
  futex: Remove bogus condition for requeue PI
  ...
2021-08-30 14:26:36 -07:00

1515 lines
48 KiB
C

/* SPDX-License-Identifier: GPL-2.0+ */
/*
* Read-Copy Update mechanism for mutual exclusion (tree-based version)
* Internal non-public definitions that provide either classic
* or preemptible semantics.
*
* Copyright Red Hat, 2009
* Copyright IBM Corporation, 2009
*
* Author: Ingo Molnar <mingo@elte.hu>
* Paul E. McKenney <paulmck@linux.ibm.com>
*/
#include "../locking/rtmutex_common.h"
static bool rcu_rdp_is_offloaded(struct rcu_data *rdp)
{
/*
* In order to read the offloaded state of an rdp is a safe
* and stable way and prevent from its value to be changed
* under us, we must either hold the barrier mutex, the cpu
* hotplug lock (read or write) or the nocb lock. Local
* non-preemptible reads are also safe. NOCB kthreads and
* timers have their own means of synchronization against the
* offloaded state updaters.
*/
RCU_LOCKDEP_WARN(
!(lockdep_is_held(&rcu_state.barrier_mutex) ||
(IS_ENABLED(CONFIG_HOTPLUG_CPU) && lockdep_is_cpus_held()) ||
rcu_lockdep_is_held_nocb(rdp) ||
(rdp == this_cpu_ptr(&rcu_data) &&
!(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible())) ||
rcu_current_is_nocb_kthread(rdp)),
"Unsafe read of RCU_NOCB offloaded state"
);
return rcu_segcblist_is_offloaded(&rdp->cblist);
}
/*
* Check the RCU kernel configuration parameters and print informative
* messages about anything out of the ordinary.
*/
static void __init rcu_bootup_announce_oddness(void)
{
if (IS_ENABLED(CONFIG_RCU_TRACE))
pr_info("\tRCU event tracing is enabled.\n");
if ((IS_ENABLED(CONFIG_64BIT) && RCU_FANOUT != 64) ||
(!IS_ENABLED(CONFIG_64BIT) && RCU_FANOUT != 32))
pr_info("\tCONFIG_RCU_FANOUT set to non-default value of %d.\n",
RCU_FANOUT);
if (rcu_fanout_exact)
pr_info("\tHierarchical RCU autobalancing is disabled.\n");
if (IS_ENABLED(CONFIG_RCU_FAST_NO_HZ))
pr_info("\tRCU dyntick-idle grace-period acceleration is enabled.\n");
if (IS_ENABLED(CONFIG_PROVE_RCU))
pr_info("\tRCU lockdep checking is enabled.\n");
if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD))
pr_info("\tRCU strict (and thus non-scalable) grace periods enabled.\n");
if (RCU_NUM_LVLS >= 4)
pr_info("\tFour(or more)-level hierarchy is enabled.\n");
if (RCU_FANOUT_LEAF != 16)
pr_info("\tBuild-time adjustment of leaf fanout to %d.\n",
RCU_FANOUT_LEAF);
if (rcu_fanout_leaf != RCU_FANOUT_LEAF)
pr_info("\tBoot-time adjustment of leaf fanout to %d.\n",
rcu_fanout_leaf);
if (nr_cpu_ids != NR_CPUS)
pr_info("\tRCU restricting CPUs from NR_CPUS=%d to nr_cpu_ids=%u.\n", NR_CPUS, nr_cpu_ids);
#ifdef CONFIG_RCU_BOOST
pr_info("\tRCU priority boosting: priority %d delay %d ms.\n",
kthread_prio, CONFIG_RCU_BOOST_DELAY);
#endif
if (blimit != DEFAULT_RCU_BLIMIT)
pr_info("\tBoot-time adjustment of callback invocation limit to %ld.\n", blimit);
if (qhimark != DEFAULT_RCU_QHIMARK)
pr_info("\tBoot-time adjustment of callback high-water mark to %ld.\n", qhimark);
if (qlowmark != DEFAULT_RCU_QLOMARK)
pr_info("\tBoot-time adjustment of callback low-water mark to %ld.\n", qlowmark);
if (qovld != DEFAULT_RCU_QOVLD)
pr_info("\tBoot-time adjustment of callback overload level to %ld.\n", qovld);
if (jiffies_till_first_fqs != ULONG_MAX)
pr_info("\tBoot-time adjustment of first FQS scan delay to %ld jiffies.\n", jiffies_till_first_fqs);
if (jiffies_till_next_fqs != ULONG_MAX)
pr_info("\tBoot-time adjustment of subsequent FQS scan delay to %ld jiffies.\n", jiffies_till_next_fqs);
if (jiffies_till_sched_qs != ULONG_MAX)
pr_info("\tBoot-time adjustment of scheduler-enlistment delay to %ld jiffies.\n", jiffies_till_sched_qs);
if (rcu_kick_kthreads)
pr_info("\tKick kthreads if too-long grace period.\n");
if (IS_ENABLED(CONFIG_DEBUG_OBJECTS_RCU_HEAD))
pr_info("\tRCU callback double-/use-after-free debug enabled.\n");
if (gp_preinit_delay)
pr_info("\tRCU debug GP pre-init slowdown %d jiffies.\n", gp_preinit_delay);
if (gp_init_delay)
pr_info("\tRCU debug GP init slowdown %d jiffies.\n", gp_init_delay);
if (gp_cleanup_delay)
pr_info("\tRCU debug GP init slowdown %d jiffies.\n", gp_cleanup_delay);
if (!use_softirq)
pr_info("\tRCU_SOFTIRQ processing moved to rcuc kthreads.\n");
if (IS_ENABLED(CONFIG_RCU_EQS_DEBUG))
pr_info("\tRCU debug extended QS entry/exit.\n");
rcupdate_announce_bootup_oddness();
}
#ifdef CONFIG_PREEMPT_RCU
static void rcu_report_exp_rnp(struct rcu_node *rnp, bool wake);
static void rcu_read_unlock_special(struct task_struct *t);
/*
* Tell them what RCU they are running.
*/
static void __init rcu_bootup_announce(void)
{
pr_info("Preemptible hierarchical RCU implementation.\n");
rcu_bootup_announce_oddness();
}
/* Flags for rcu_preempt_ctxt_queue() decision table. */
#define RCU_GP_TASKS 0x8
#define RCU_EXP_TASKS 0x4
#define RCU_GP_BLKD 0x2
#define RCU_EXP_BLKD 0x1
/*
* Queues a task preempted within an RCU-preempt read-side critical
* section into the appropriate location within the ->blkd_tasks list,
* depending on the states of any ongoing normal and expedited grace
* periods. The ->gp_tasks pointer indicates which element the normal
* grace period is waiting on (NULL if none), and the ->exp_tasks pointer
* indicates which element the expedited grace period is waiting on (again,
* NULL if none). If a grace period is waiting on a given element in the
* ->blkd_tasks list, it also waits on all subsequent elements. Thus,
* adding a task to the tail of the list blocks any grace period that is
* already waiting on one of the elements. In contrast, adding a task
* to the head of the list won't block any grace period that is already
* waiting on one of the elements.
*
* This queuing is imprecise, and can sometimes make an ongoing grace
* period wait for a task that is not strictly speaking blocking it.
* Given the choice, we needlessly block a normal grace period rather than
* blocking an expedited grace period.
*
* Note that an endless sequence of expedited grace periods still cannot
* indefinitely postpone a normal grace period. Eventually, all of the
* fixed number of preempted tasks blocking the normal grace period that are
* not also blocking the expedited grace period will resume and complete
* their RCU read-side critical sections. At that point, the ->gp_tasks
* pointer will equal the ->exp_tasks pointer, at which point the end of
* the corresponding expedited grace period will also be the end of the
* normal grace period.
*/
static void rcu_preempt_ctxt_queue(struct rcu_node *rnp, struct rcu_data *rdp)
__releases(rnp->lock) /* But leaves rrupts disabled. */
{
int blkd_state = (rnp->gp_tasks ? RCU_GP_TASKS : 0) +
(rnp->exp_tasks ? RCU_EXP_TASKS : 0) +
(rnp->qsmask & rdp->grpmask ? RCU_GP_BLKD : 0) +
(rnp->expmask & rdp->grpmask ? RCU_EXP_BLKD : 0);
struct task_struct *t = current;
raw_lockdep_assert_held_rcu_node(rnp);
WARN_ON_ONCE(rdp->mynode != rnp);
WARN_ON_ONCE(!rcu_is_leaf_node(rnp));
/* RCU better not be waiting on newly onlined CPUs! */
WARN_ON_ONCE(rnp->qsmaskinitnext & ~rnp->qsmaskinit & rnp->qsmask &
rdp->grpmask);
/*
* Decide where to queue the newly blocked task. In theory,
* this could be an if-statement. In practice, when I tried
* that, it was quite messy.
*/
switch (blkd_state) {
case 0:
case RCU_EXP_TASKS:
case RCU_EXP_TASKS + RCU_GP_BLKD:
case RCU_GP_TASKS:
case RCU_GP_TASKS + RCU_EXP_TASKS:
/*
* Blocking neither GP, or first task blocking the normal
* GP but not blocking the already-waiting expedited GP.
* Queue at the head of the list to avoid unnecessarily
* blocking the already-waiting GPs.
*/
list_add(&t->rcu_node_entry, &rnp->blkd_tasks);
break;
case RCU_EXP_BLKD:
case RCU_GP_BLKD:
case RCU_GP_BLKD + RCU_EXP_BLKD:
case RCU_GP_TASKS + RCU_EXP_BLKD:
case RCU_GP_TASKS + RCU_GP_BLKD + RCU_EXP_BLKD:
case RCU_GP_TASKS + RCU_EXP_TASKS + RCU_GP_BLKD + RCU_EXP_BLKD:
/*
* First task arriving that blocks either GP, or first task
* arriving that blocks the expedited GP (with the normal
* GP already waiting), or a task arriving that blocks
* both GPs with both GPs already waiting. Queue at the
* tail of the list to avoid any GP waiting on any of the
* already queued tasks that are not blocking it.
*/
list_add_tail(&t->rcu_node_entry, &rnp->blkd_tasks);
break;
case RCU_EXP_TASKS + RCU_EXP_BLKD:
case RCU_EXP_TASKS + RCU_GP_BLKD + RCU_EXP_BLKD:
case RCU_GP_TASKS + RCU_EXP_TASKS + RCU_EXP_BLKD:
/*
* Second or subsequent task blocking the expedited GP.
* The task either does not block the normal GP, or is the
* first task blocking the normal GP. Queue just after
* the first task blocking the expedited GP.
*/
list_add(&t->rcu_node_entry, rnp->exp_tasks);
break;
case RCU_GP_TASKS + RCU_GP_BLKD:
case RCU_GP_TASKS + RCU_EXP_TASKS + RCU_GP_BLKD:
/*
* Second or subsequent task blocking the normal GP.
* The task does not block the expedited GP. Queue just
* after the first task blocking the normal GP.
*/
list_add(&t->rcu_node_entry, rnp->gp_tasks);
break;
default:
/* Yet another exercise in excessive paranoia. */
WARN_ON_ONCE(1);
break;
}
/*
* We have now queued the task. If it was the first one to
* block either grace period, update the ->gp_tasks and/or
* ->exp_tasks pointers, respectively, to reference the newly
* blocked tasks.
*/
if (!rnp->gp_tasks && (blkd_state & RCU_GP_BLKD)) {
WRITE_ONCE(rnp->gp_tasks, &t->rcu_node_entry);
WARN_ON_ONCE(rnp->completedqs == rnp->gp_seq);
}
if (!rnp->exp_tasks && (blkd_state & RCU_EXP_BLKD))
WRITE_ONCE(rnp->exp_tasks, &t->rcu_node_entry);
WARN_ON_ONCE(!(blkd_state & RCU_GP_BLKD) !=
!(rnp->qsmask & rdp->grpmask));
WARN_ON_ONCE(!(blkd_state & RCU_EXP_BLKD) !=
!(rnp->expmask & rdp->grpmask));
raw_spin_unlock_rcu_node(rnp); /* interrupts remain disabled. */
/*
* Report the quiescent state for the expedited GP. This expedited
* GP should not be able to end until we report, so there should be
* no need to check for a subsequent expedited GP. (Though we are
* still in a quiescent state in any case.)
*/
if (blkd_state & RCU_EXP_BLKD && rdp->exp_deferred_qs)
rcu_report_exp_rdp(rdp);
else
WARN_ON_ONCE(rdp->exp_deferred_qs);
}
/*
* Record a preemptible-RCU quiescent state for the specified CPU.
* Note that this does not necessarily mean that the task currently running
* on the CPU is in a quiescent state: Instead, it means that the current
* grace period need not wait on any RCU read-side critical section that
* starts later on this CPU. It also means that if the current task is
* in an RCU read-side critical section, it has already added itself to
* some leaf rcu_node structure's ->blkd_tasks list. In addition to the
* current task, there might be any number of other tasks blocked while
* in an RCU read-side critical section.
*
* Callers to this function must disable preemption.
*/
static void rcu_qs(void)
{
RCU_LOCKDEP_WARN(preemptible(), "rcu_qs() invoked with preemption enabled!!!\n");
if (__this_cpu_read(rcu_data.cpu_no_qs.s)) {
trace_rcu_grace_period(TPS("rcu_preempt"),
__this_cpu_read(rcu_data.gp_seq),
TPS("cpuqs"));
__this_cpu_write(rcu_data.cpu_no_qs.b.norm, false);
barrier(); /* Coordinate with rcu_flavor_sched_clock_irq(). */
WRITE_ONCE(current->rcu_read_unlock_special.b.need_qs, false);
}
}
/*
* We have entered the scheduler, and the current task might soon be
* context-switched away from. If this task is in an RCU read-side
* critical section, we will no longer be able to rely on the CPU to
* record that fact, so we enqueue the task on the blkd_tasks list.
* The task will dequeue itself when it exits the outermost enclosing
* RCU read-side critical section. Therefore, the current grace period
* cannot be permitted to complete until the blkd_tasks list entries
* predating the current grace period drain, in other words, until
* rnp->gp_tasks becomes NULL.
*
* Caller must disable interrupts.
*/
void rcu_note_context_switch(bool preempt)
{
struct task_struct *t = current;
struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
struct rcu_node *rnp;
trace_rcu_utilization(TPS("Start context switch"));
lockdep_assert_irqs_disabled();
WARN_ONCE(!preempt && rcu_preempt_depth() > 0, "Voluntary context switch within RCU read-side critical section!");
if (rcu_preempt_depth() > 0 &&
!t->rcu_read_unlock_special.b.blocked) {
/* Possibly blocking in an RCU read-side critical section. */
rnp = rdp->mynode;
raw_spin_lock_rcu_node(rnp);
t->rcu_read_unlock_special.b.blocked = true;
t->rcu_blocked_node = rnp;
/*
* Verify the CPU's sanity, trace the preemption, and
* then queue the task as required based on the states
* of any ongoing and expedited grace periods.
*/
WARN_ON_ONCE((rdp->grpmask & rcu_rnp_online_cpus(rnp)) == 0);
WARN_ON_ONCE(!list_empty(&t->rcu_node_entry));
trace_rcu_preempt_task(rcu_state.name,
t->pid,
(rnp->qsmask & rdp->grpmask)
? rnp->gp_seq
: rcu_seq_snap(&rnp->gp_seq));
rcu_preempt_ctxt_queue(rnp, rdp);
} else {
rcu_preempt_deferred_qs(t);
}
/*
* Either we were not in an RCU read-side critical section to
* begin with, or we have now recorded that critical section
* globally. Either way, we can now note a quiescent state
* for this CPU. Again, if we were in an RCU read-side critical
* section, and if that critical section was blocking the current
* grace period, then the fact that the task has been enqueued
* means that we continue to block the current grace period.
*/
rcu_qs();
if (rdp->exp_deferred_qs)
rcu_report_exp_rdp(rdp);
rcu_tasks_qs(current, preempt);
trace_rcu_utilization(TPS("End context switch"));
}
EXPORT_SYMBOL_GPL(rcu_note_context_switch);
/*
* Check for preempted RCU readers blocking the current grace period
* for the specified rcu_node structure. If the caller needs a reliable
* answer, it must hold the rcu_node's ->lock.
*/
static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
{
return READ_ONCE(rnp->gp_tasks) != NULL;
}
/* limit value for ->rcu_read_lock_nesting. */
#define RCU_NEST_PMAX (INT_MAX / 2)
static void rcu_preempt_read_enter(void)
{
WRITE_ONCE(current->rcu_read_lock_nesting, READ_ONCE(current->rcu_read_lock_nesting) + 1);
}
static int rcu_preempt_read_exit(void)
{
int ret = READ_ONCE(current->rcu_read_lock_nesting) - 1;
WRITE_ONCE(current->rcu_read_lock_nesting, ret);
return ret;
}
static void rcu_preempt_depth_set(int val)
{
WRITE_ONCE(current->rcu_read_lock_nesting, val);
}
/*
* Preemptible RCU implementation for rcu_read_lock().
* Just increment ->rcu_read_lock_nesting, shared state will be updated
* if we block.
*/
void __rcu_read_lock(void)
{
rcu_preempt_read_enter();
if (IS_ENABLED(CONFIG_PROVE_LOCKING))
WARN_ON_ONCE(rcu_preempt_depth() > RCU_NEST_PMAX);
if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) && rcu_state.gp_kthread)
WRITE_ONCE(current->rcu_read_unlock_special.b.need_qs, true);
barrier(); /* critical section after entry code. */
}
EXPORT_SYMBOL_GPL(__rcu_read_lock);
/*
* Preemptible RCU implementation for rcu_read_unlock().
* Decrement ->rcu_read_lock_nesting. If the result is zero (outermost
* rcu_read_unlock()) and ->rcu_read_unlock_special is non-zero, then
* invoke rcu_read_unlock_special() to clean up after a context switch
* in an RCU read-side critical section and other special cases.
*/
void __rcu_read_unlock(void)
{
struct task_struct *t = current;
barrier(); // critical section before exit code.
if (rcu_preempt_read_exit() == 0) {
barrier(); // critical-section exit before .s check.
if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
rcu_read_unlock_special(t);
}
if (IS_ENABLED(CONFIG_PROVE_LOCKING)) {
int rrln = rcu_preempt_depth();
WARN_ON_ONCE(rrln < 0 || rrln > RCU_NEST_PMAX);
}
}
EXPORT_SYMBOL_GPL(__rcu_read_unlock);
/*
* Advance a ->blkd_tasks-list pointer to the next entry, instead
* returning NULL if at the end of the list.
*/
static struct list_head *rcu_next_node_entry(struct task_struct *t,
struct rcu_node *rnp)
{
struct list_head *np;
np = t->rcu_node_entry.next;
if (np == &rnp->blkd_tasks)
np = NULL;
return np;
}
/*
* Return true if the specified rcu_node structure has tasks that were
* preempted within an RCU read-side critical section.
*/
static bool rcu_preempt_has_tasks(struct rcu_node *rnp)
{
return !list_empty(&rnp->blkd_tasks);
}
/*
* Report deferred quiescent states. The deferral time can
* be quite short, for example, in the case of the call from
* rcu_read_unlock_special().
*/
static void
rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
{
bool empty_exp;
bool empty_norm;
bool empty_exp_now;
struct list_head *np;
bool drop_boost_mutex = false;
struct rcu_data *rdp;
struct rcu_node *rnp;
union rcu_special special;
/*
* If RCU core is waiting for this CPU to exit its critical section,
* report the fact that it has exited. Because irqs are disabled,
* t->rcu_read_unlock_special cannot change.
*/
special = t->rcu_read_unlock_special;
rdp = this_cpu_ptr(&rcu_data);
if (!special.s && !rdp->exp_deferred_qs) {
local_irq_restore(flags);
return;
}
t->rcu_read_unlock_special.s = 0;
if (special.b.need_qs) {
if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD)) {
rcu_report_qs_rdp(rdp);
udelay(rcu_unlock_delay);
} else {
rcu_qs();
}
}
/*
* Respond to a request by an expedited grace period for a
* quiescent state from this CPU. Note that requests from
* tasks are handled when removing the task from the
* blocked-tasks list below.
*/
if (rdp->exp_deferred_qs)
rcu_report_exp_rdp(rdp);
/* Clean up if blocked during RCU read-side critical section. */
if (special.b.blocked) {
/*
* Remove this task from the list it blocked on. The task
* now remains queued on the rcu_node corresponding to the
* CPU it first blocked on, so there is no longer any need
* to loop. Retain a WARN_ON_ONCE() out of sheer paranoia.
*/
rnp = t->rcu_blocked_node;
raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
WARN_ON_ONCE(rnp != t->rcu_blocked_node);
WARN_ON_ONCE(!rcu_is_leaf_node(rnp));
empty_norm = !rcu_preempt_blocked_readers_cgp(rnp);
WARN_ON_ONCE(rnp->completedqs == rnp->gp_seq &&
(!empty_norm || rnp->qsmask));
empty_exp = sync_rcu_exp_done(rnp);
smp_mb(); /* ensure expedited fastpath sees end of RCU c-s. */
np = rcu_next_node_entry(t, rnp);
list_del_init(&t->rcu_node_entry);
t->rcu_blocked_node = NULL;
trace_rcu_unlock_preempted_task(TPS("rcu_preempt"),
rnp->gp_seq, t->pid);
if (&t->rcu_node_entry == rnp->gp_tasks)
WRITE_ONCE(rnp->gp_tasks, np);
if (&t->rcu_node_entry == rnp->exp_tasks)
WRITE_ONCE(rnp->exp_tasks, np);
if (IS_ENABLED(CONFIG_RCU_BOOST)) {
/* Snapshot ->boost_mtx ownership w/rnp->lock held. */
drop_boost_mutex = rt_mutex_owner(&rnp->boost_mtx.rtmutex) == t;
if (&t->rcu_node_entry == rnp->boost_tasks)
WRITE_ONCE(rnp->boost_tasks, np);
}
/*
* If this was the last task on the current list, and if
* we aren't waiting on any CPUs, report the quiescent state.
* Note that rcu_report_unblock_qs_rnp() releases rnp->lock,
* so we must take a snapshot of the expedited state.
*/
empty_exp_now = sync_rcu_exp_done(rnp);
if (!empty_norm && !rcu_preempt_blocked_readers_cgp(rnp)) {
trace_rcu_quiescent_state_report(TPS("preempt_rcu"),
rnp->gp_seq,
0, rnp->qsmask,
rnp->level,
rnp->grplo,
rnp->grphi,
!!rnp->gp_tasks);
rcu_report_unblock_qs_rnp(rnp, flags);
} else {
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
}
/* Unboost if we were boosted. */
if (IS_ENABLED(CONFIG_RCU_BOOST) && drop_boost_mutex)
rt_mutex_futex_unlock(&rnp->boost_mtx.rtmutex);
/*
* If this was the last task on the expedited lists,
* then we need to report up the rcu_node hierarchy.
*/
if (!empty_exp && empty_exp_now)
rcu_report_exp_rnp(rnp, true);
} else {
local_irq_restore(flags);
}
}
/*
* Is a deferred quiescent-state pending, and are we also not in
* an RCU read-side critical section? It is the caller's responsibility
* to ensure it is otherwise safe to report any deferred quiescent
* states. The reason for this is that it is safe to report a
* quiescent state during context switch even though preemption
* is disabled. This function cannot be expected to understand these
* nuances, so the caller must handle them.
*/
static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
{
return (__this_cpu_read(rcu_data.exp_deferred_qs) ||
READ_ONCE(t->rcu_read_unlock_special.s)) &&
rcu_preempt_depth() == 0;
}
/*
* Report a deferred quiescent state if needed and safe to do so.
* As with rcu_preempt_need_deferred_qs(), "safe" involves only
* not being in an RCU read-side critical section. The caller must
* evaluate safety in terms of interrupt, softirq, and preemption
* disabling.
*/
static void rcu_preempt_deferred_qs(struct task_struct *t)
{
unsigned long flags;
if (!rcu_preempt_need_deferred_qs(t))
return;
local_irq_save(flags);
rcu_preempt_deferred_qs_irqrestore(t, flags);
}
/*
* Minimal handler to give the scheduler a chance to re-evaluate.
*/
static void rcu_preempt_deferred_qs_handler(struct irq_work *iwp)
{
struct rcu_data *rdp;
rdp = container_of(iwp, struct rcu_data, defer_qs_iw);
rdp->defer_qs_iw_pending = false;
}
/*
* Handle special cases during rcu_read_unlock(), such as needing to
* notify RCU core processing or task having blocked during the RCU
* read-side critical section.
*/
static void rcu_read_unlock_special(struct task_struct *t)
{
unsigned long flags;
bool irqs_were_disabled;
bool preempt_bh_were_disabled =
!!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK));
/* NMI handlers cannot block and cannot safely manipulate state. */
if (in_nmi())
return;
local_irq_save(flags);
irqs_were_disabled = irqs_disabled_flags(flags);
if (preempt_bh_were_disabled || irqs_were_disabled) {
bool expboost; // Expedited GP in flight or possible boosting.
struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
struct rcu_node *rnp = rdp->mynode;
expboost = (t->rcu_blocked_node && READ_ONCE(t->rcu_blocked_node->exp_tasks)) ||
(rdp->grpmask & READ_ONCE(rnp->expmask)) ||
IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) ||
(IS_ENABLED(CONFIG_RCU_BOOST) && irqs_were_disabled &&
t->rcu_blocked_node);
// Need to defer quiescent state until everything is enabled.
if (use_softirq && (in_irq() || (expboost && !irqs_were_disabled))) {
// Using softirq, safe to awaken, and either the
// wakeup is free or there is either an expedited
// GP in flight or a potential need to deboost.
raise_softirq_irqoff(RCU_SOFTIRQ);
} else {
// Enabling BH or preempt does reschedule, so...
// Also if no expediting and no possible deboosting,
// slow is OK. Plus nohz_full CPUs eventually get
// tick enabled.
set_tsk_need_resched(current);
set_preempt_need_resched();
if (IS_ENABLED(CONFIG_IRQ_WORK) && irqs_were_disabled &&
expboost && !rdp->defer_qs_iw_pending && cpu_online(rdp->cpu)) {
// Get scheduler to re-evaluate and call hooks.
// If !IRQ_WORK, FQS scan will eventually IPI.
init_irq_work(&rdp->defer_qs_iw, rcu_preempt_deferred_qs_handler);
rdp->defer_qs_iw_pending = true;
irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu);
}
}
local_irq_restore(flags);
return;
}
rcu_preempt_deferred_qs_irqrestore(t, flags);
}
/*
* Check that the list of blocked tasks for the newly completed grace
* period is in fact empty. It is a serious bug to complete a grace
* period that still has RCU readers blocked! This function must be
* invoked -before- updating this rnp's ->gp_seq.
*
* Also, if there are blocked tasks on the list, they automatically
* block the newly created grace period, so set up ->gp_tasks accordingly.
*/
static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
{
struct task_struct *t;
RCU_LOCKDEP_WARN(preemptible(), "rcu_preempt_check_blocked_tasks() invoked with preemption enabled!!!\n");
raw_lockdep_assert_held_rcu_node(rnp);
if (WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp)))
dump_blkd_tasks(rnp, 10);
if (rcu_preempt_has_tasks(rnp) &&
(rnp->qsmaskinit || rnp->wait_blkd_tasks)) {
WRITE_ONCE(rnp->gp_tasks, rnp->blkd_tasks.next);
t = container_of(rnp->gp_tasks, struct task_struct,
rcu_node_entry);
trace_rcu_unlock_preempted_task(TPS("rcu_preempt-GPS"),
rnp->gp_seq, t->pid);
}
WARN_ON_ONCE(rnp->qsmask);
}
/*
* Check for a quiescent state from the current CPU, including voluntary
* context switches for Tasks RCU. When a task blocks, the task is
* recorded in the corresponding CPU's rcu_node structure, which is checked
* elsewhere, hence this function need only check for quiescent states
* related to the current CPU, not to those related to tasks.
*/
static void rcu_flavor_sched_clock_irq(int user)
{
struct task_struct *t = current;
lockdep_assert_irqs_disabled();
if (user || rcu_is_cpu_rrupt_from_idle()) {
rcu_note_voluntary_context_switch(current);
}
if (rcu_preempt_depth() > 0 ||
(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK))) {
/* No QS, force context switch if deferred. */
if (rcu_preempt_need_deferred_qs(t)) {
set_tsk_need_resched(t);
set_preempt_need_resched();
}
} else if (rcu_preempt_need_deferred_qs(t)) {
rcu_preempt_deferred_qs(t); /* Report deferred QS. */
return;
} else if (!WARN_ON_ONCE(rcu_preempt_depth())) {
rcu_qs(); /* Report immediate QS. */
return;
}
/* If GP is oldish, ask for help from rcu_read_unlock_special(). */
if (rcu_preempt_depth() > 0 &&
__this_cpu_read(rcu_data.core_needs_qs) &&
__this_cpu_read(rcu_data.cpu_no_qs.b.norm) &&
!t->rcu_read_unlock_special.b.need_qs &&
time_after(jiffies, rcu_state.gp_start + HZ))
t->rcu_read_unlock_special.b.need_qs = true;
}
/*
* Check for a task exiting while in a preemptible-RCU read-side
* critical section, clean up if so. No need to issue warnings, as
* debug_check_no_locks_held() already does this if lockdep is enabled.
* Besides, if this function does anything other than just immediately
* return, there was a bug of some sort. Spewing warnings from this
* function is like as not to simply obscure important prior warnings.
*/
void exit_rcu(void)
{
struct task_struct *t = current;
if (unlikely(!list_empty(&current->rcu_node_entry))) {
rcu_preempt_depth_set(1);
barrier();
WRITE_ONCE(t->rcu_read_unlock_special.b.blocked, true);
} else if (unlikely(rcu_preempt_depth())) {
rcu_preempt_depth_set(1);
} else {
return;
}
__rcu_read_unlock();
rcu_preempt_deferred_qs(current);
}
/*
* Dump the blocked-tasks state, but limit the list dump to the
* specified number of elements.
*/
static void
dump_blkd_tasks(struct rcu_node *rnp, int ncheck)
{
int cpu;
int i;
struct list_head *lhp;
bool onl;
struct rcu_data *rdp;
struct rcu_node *rnp1;
raw_lockdep_assert_held_rcu_node(rnp);
pr_info("%s: grp: %d-%d level: %d ->gp_seq %ld ->completedqs %ld\n",
__func__, rnp->grplo, rnp->grphi, rnp->level,
(long)READ_ONCE(rnp->gp_seq), (long)rnp->completedqs);
for (rnp1 = rnp; rnp1; rnp1 = rnp1->parent)
pr_info("%s: %d:%d ->qsmask %#lx ->qsmaskinit %#lx ->qsmaskinitnext %#lx\n",
__func__, rnp1->grplo, rnp1->grphi, rnp1->qsmask, rnp1->qsmaskinit, rnp1->qsmaskinitnext);
pr_info("%s: ->gp_tasks %p ->boost_tasks %p ->exp_tasks %p\n",
__func__, READ_ONCE(rnp->gp_tasks), data_race(rnp->boost_tasks),
READ_ONCE(rnp->exp_tasks));
pr_info("%s: ->blkd_tasks", __func__);
i = 0;
list_for_each(lhp, &rnp->blkd_tasks) {
pr_cont(" %p", lhp);
if (++i >= ncheck)
break;
}
pr_cont("\n");
for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++) {
rdp = per_cpu_ptr(&rcu_data, cpu);
onl = !!(rdp->grpmask & rcu_rnp_online_cpus(rnp));
pr_info("\t%d: %c online: %ld(%d) offline: %ld(%d)\n",
cpu, ".o"[onl],
(long)rdp->rcu_onl_gp_seq, rdp->rcu_onl_gp_flags,
(long)rdp->rcu_ofl_gp_seq, rdp->rcu_ofl_gp_flags);
}
}
#else /* #ifdef CONFIG_PREEMPT_RCU */
/*
* If strict grace periods are enabled, and if the calling
* __rcu_read_unlock() marks the beginning of a quiescent state, immediately
* report that quiescent state and, if requested, spin for a bit.
*/
void rcu_read_unlock_strict(void)
{
struct rcu_data *rdp;
if (!IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) ||
irqs_disabled() || preempt_count() || !rcu_state.gp_kthread)
return;
rdp = this_cpu_ptr(&rcu_data);
rcu_report_qs_rdp(rdp);
udelay(rcu_unlock_delay);
}
EXPORT_SYMBOL_GPL(rcu_read_unlock_strict);
/*
* Tell them what RCU they are running.
*/
static void __init rcu_bootup_announce(void)
{
pr_info("Hierarchical RCU implementation.\n");
rcu_bootup_announce_oddness();
}
/*
* Note a quiescent state for PREEMPTION=n. Because we do not need to know
* how many quiescent states passed, just if there was at least one since
* the start of the grace period, this just sets a flag. The caller must
* have disabled preemption.
*/
static void rcu_qs(void)
{
RCU_LOCKDEP_WARN(preemptible(), "rcu_qs() invoked with preemption enabled!!!");
if (!__this_cpu_read(rcu_data.cpu_no_qs.s))
return;
trace_rcu_grace_period(TPS("rcu_sched"),
__this_cpu_read(rcu_data.gp_seq), TPS("cpuqs"));
__this_cpu_write(rcu_data.cpu_no_qs.b.norm, false);
if (!__this_cpu_read(rcu_data.cpu_no_qs.b.exp))
return;
__this_cpu_write(rcu_data.cpu_no_qs.b.exp, false);
rcu_report_exp_rdp(this_cpu_ptr(&rcu_data));
}
/*
* Register an urgently needed quiescent state. If there is an
* emergency, invoke rcu_momentary_dyntick_idle() to do a heavy-weight
* dyntick-idle quiescent state visible to other CPUs, which will in
* some cases serve for expedited as well as normal grace periods.
* Either way, register a lightweight quiescent state.
*/
void rcu_all_qs(void)
{
unsigned long flags;
if (!raw_cpu_read(rcu_data.rcu_urgent_qs))
return;
preempt_disable();
/* Load rcu_urgent_qs before other flags. */
if (!smp_load_acquire(this_cpu_ptr(&rcu_data.rcu_urgent_qs))) {
preempt_enable();
return;
}
this_cpu_write(rcu_data.rcu_urgent_qs, false);
if (unlikely(raw_cpu_read(rcu_data.rcu_need_heavy_qs))) {
local_irq_save(flags);
rcu_momentary_dyntick_idle();
local_irq_restore(flags);
}
rcu_qs();
preempt_enable();
}
EXPORT_SYMBOL_GPL(rcu_all_qs);
/*
* Note a PREEMPTION=n context switch. The caller must have disabled interrupts.
*/
void rcu_note_context_switch(bool preempt)
{
trace_rcu_utilization(TPS("Start context switch"));
rcu_qs();
/* Load rcu_urgent_qs before other flags. */
if (!smp_load_acquire(this_cpu_ptr(&rcu_data.rcu_urgent_qs)))
goto out;
this_cpu_write(rcu_data.rcu_urgent_qs, false);
if (unlikely(raw_cpu_read(rcu_data.rcu_need_heavy_qs)))
rcu_momentary_dyntick_idle();
rcu_tasks_qs(current, preempt);
out:
trace_rcu_utilization(TPS("End context switch"));
}
EXPORT_SYMBOL_GPL(rcu_note_context_switch);
/*
* Because preemptible RCU does not exist, there are never any preempted
* RCU readers.
*/
static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
{
return 0;
}
/*
* Because there is no preemptible RCU, there can be no readers blocked.
*/
static bool rcu_preempt_has_tasks(struct rcu_node *rnp)
{
return false;
}
/*
* Because there is no preemptible RCU, there can be no deferred quiescent
* states.
*/
static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
{
return false;
}
static void rcu_preempt_deferred_qs(struct task_struct *t) { }
/*
* Because there is no preemptible RCU, there can be no readers blocked,
* so there is no need to check for blocked tasks. So check only for
* bogus qsmask values.
*/
static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
{
WARN_ON_ONCE(rnp->qsmask);
}
/*
* Check to see if this CPU is in a non-context-switch quiescent state,
* namely user mode and idle loop.
*/
static void rcu_flavor_sched_clock_irq(int user)
{
if (user || rcu_is_cpu_rrupt_from_idle()) {
/*
* Get here if this CPU took its interrupt from user
* mode or from the idle loop, and if this is not a
* nested interrupt. In this case, the CPU is in
* a quiescent state, so note it.
*
* No memory barrier is required here because rcu_qs()
* references only CPU-local variables that other CPUs
* neither access nor modify, at least not while the
* corresponding CPU is online.
*/
rcu_qs();
}
}
/*
* Because preemptible RCU does not exist, tasks cannot possibly exit
* while in preemptible RCU read-side critical sections.
*/
void exit_rcu(void)
{
}
/*
* Dump the guaranteed-empty blocked-tasks state. Trust but verify.
*/
static void
dump_blkd_tasks(struct rcu_node *rnp, int ncheck)
{
WARN_ON_ONCE(!list_empty(&rnp->blkd_tasks));
}
#endif /* #else #ifdef CONFIG_PREEMPT_RCU */
/*
* If boosting, set rcuc kthreads to realtime priority.
*/
static void rcu_cpu_kthread_setup(unsigned int cpu)
{
#ifdef CONFIG_RCU_BOOST
struct sched_param sp;
sp.sched_priority = kthread_prio;
sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);
#endif /* #ifdef CONFIG_RCU_BOOST */
}
#ifdef CONFIG_RCU_BOOST
/*
* Carry out RCU priority boosting on the task indicated by ->exp_tasks
* or ->boost_tasks, advancing the pointer to the next task in the
* ->blkd_tasks list.
*
* Note that irqs must be enabled: boosting the task can block.
* Returns 1 if there are more tasks needing to be boosted.
*/
static int rcu_boost(struct rcu_node *rnp)
{
unsigned long flags;
struct task_struct *t;
struct list_head *tb;
if (READ_ONCE(rnp->exp_tasks) == NULL &&
READ_ONCE(rnp->boost_tasks) == NULL)
return 0; /* Nothing left to boost. */
raw_spin_lock_irqsave_rcu_node(rnp, flags);
/*
* Recheck under the lock: all tasks in need of boosting
* might exit their RCU read-side critical sections on their own.
*/
if (rnp->exp_tasks == NULL && rnp->boost_tasks == NULL) {
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
return 0;
}
/*
* Preferentially boost tasks blocking expedited grace periods.
* This cannot starve the normal grace periods because a second
* expedited grace period must boost all blocked tasks, including
* those blocking the pre-existing normal grace period.
*/
if (rnp->exp_tasks != NULL)
tb = rnp->exp_tasks;
else
tb = rnp->boost_tasks;
/*
* We boost task t by manufacturing an rt_mutex that appears to
* be held by task t. We leave a pointer to that rt_mutex where
* task t can find it, and task t will release the mutex when it
* exits its outermost RCU read-side critical section. Then
* simply acquiring this artificial rt_mutex will boost task
* t's priority. (Thanks to tglx for suggesting this approach!)
*
* Note that task t must acquire rnp->lock to remove itself from
* the ->blkd_tasks list, which it will do from exit() if from
* nowhere else. We therefore are guaranteed that task t will
* stay around at least until we drop rnp->lock. Note that
* rnp->lock also resolves races between our priority boosting
* and task t's exiting its outermost RCU read-side critical
* section.
*/
t = container_of(tb, struct task_struct, rcu_node_entry);
rt_mutex_init_proxy_locked(&rnp->boost_mtx.rtmutex, t);
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
/* Lock only for side effect: boosts task t's priority. */
rt_mutex_lock(&rnp->boost_mtx);
rt_mutex_unlock(&rnp->boost_mtx); /* Then keep lockdep happy. */
rnp->n_boosts++;
return READ_ONCE(rnp->exp_tasks) != NULL ||
READ_ONCE(rnp->boost_tasks) != NULL;
}
/*
* Priority-boosting kthread, one per leaf rcu_node.
*/
static int rcu_boost_kthread(void *arg)
{
struct rcu_node *rnp = (struct rcu_node *)arg;
int spincnt = 0;
int more2boost;
trace_rcu_utilization(TPS("Start boost kthread@init"));
for (;;) {
WRITE_ONCE(rnp->boost_kthread_status, RCU_KTHREAD_WAITING);
trace_rcu_utilization(TPS("End boost kthread@rcu_wait"));
rcu_wait(READ_ONCE(rnp->boost_tasks) ||
READ_ONCE(rnp->exp_tasks));
trace_rcu_utilization(TPS("Start boost kthread@rcu_wait"));
WRITE_ONCE(rnp->boost_kthread_status, RCU_KTHREAD_RUNNING);
more2boost = rcu_boost(rnp);
if (more2boost)
spincnt++;
else
spincnt = 0;
if (spincnt > 10) {
WRITE_ONCE(rnp->boost_kthread_status, RCU_KTHREAD_YIELDING);
trace_rcu_utilization(TPS("End boost kthread@rcu_yield"));
schedule_timeout_idle(2);
trace_rcu_utilization(TPS("Start boost kthread@rcu_yield"));
spincnt = 0;
}
}
/* NOTREACHED */
trace_rcu_utilization(TPS("End boost kthread@notreached"));
return 0;
}
/*
* Check to see if it is time to start boosting RCU readers that are
* blocking the current grace period, and, if so, tell the per-rcu_node
* kthread to start boosting them. If there is an expedited grace
* period in progress, it is always time to boost.
*
* The caller must hold rnp->lock, which this function releases.
* The ->boost_kthread_task is immortal, so we don't need to worry
* about it going away.
*/
static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
__releases(rnp->lock)
{
raw_lockdep_assert_held_rcu_node(rnp);
if (!rcu_preempt_blocked_readers_cgp(rnp) && rnp->exp_tasks == NULL) {
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
return;
}
if (rnp->exp_tasks != NULL ||
(rnp->gp_tasks != NULL &&
rnp->boost_tasks == NULL &&
rnp->qsmask == 0 &&
(!time_after(rnp->boost_time, jiffies) || rcu_state.cbovld))) {
if (rnp->exp_tasks == NULL)
WRITE_ONCE(rnp->boost_tasks, rnp->gp_tasks);
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
rcu_wake_cond(rnp->boost_kthread_task,
READ_ONCE(rnp->boost_kthread_status));
} else {
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
}
}
/*
* Is the current CPU running the RCU-callbacks kthread?
* Caller must have preemption disabled.
*/
static bool rcu_is_callbacks_kthread(void)
{
return __this_cpu_read(rcu_data.rcu_cpu_kthread_task) == current;
}
#define RCU_BOOST_DELAY_JIFFIES DIV_ROUND_UP(CONFIG_RCU_BOOST_DELAY * HZ, 1000)
/*
* Do priority-boost accounting for the start of a new grace period.
*/
static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
{
rnp->boost_time = jiffies + RCU_BOOST_DELAY_JIFFIES;
}
/*
* Create an RCU-boost kthread for the specified node if one does not
* already exist. We only create this kthread for preemptible RCU.
* Returns zero if all is well, a negated errno otherwise.
*/
static void rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
{
unsigned long flags;
int rnp_index = rnp - rcu_get_root();
struct sched_param sp;
struct task_struct *t;
if (rnp->boost_kthread_task || !rcu_scheduler_fully_active)
return;
rcu_state.boost = 1;
t = kthread_create(rcu_boost_kthread, (void *)rnp,
"rcub/%d", rnp_index);
if (WARN_ON_ONCE(IS_ERR(t)))
return;
raw_spin_lock_irqsave_rcu_node(rnp, flags);
rnp->boost_kthread_task = t;
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
sp.sched_priority = kthread_prio;
sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
wake_up_process(t); /* get to TASK_INTERRUPTIBLE quickly. */
}
/*
* Set the per-rcu_node kthread's affinity to cover all CPUs that are
* served by the rcu_node in question. The CPU hotplug lock is still
* held, so the value of rnp->qsmaskinit will be stable.
*
* We don't include outgoingcpu in the affinity set, use -1 if there is
* no outgoing CPU. If there are no CPUs left in the affinity set,
* this function allows the kthread to execute on any CPU.
*/
static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
{
struct task_struct *t = rnp->boost_kthread_task;
unsigned long mask = rcu_rnp_online_cpus(rnp);
cpumask_var_t cm;
int cpu;
if (!t)
return;
if (!zalloc_cpumask_var(&cm, GFP_KERNEL))
return;
for_each_leaf_node_possible_cpu(rnp, cpu)
if ((mask & leaf_node_cpu_bit(rnp, cpu)) &&
cpu != outgoingcpu)
cpumask_set_cpu(cpu, cm);
if (cpumask_weight(cm) == 0)
cpumask_setall(cm);
set_cpus_allowed_ptr(t, cm);
free_cpumask_var(cm);
}
/*
* Spawn boost kthreads -- called as soon as the scheduler is running.
*/
static void __init rcu_spawn_boost_kthreads(void)
{
struct rcu_node *rnp;
rcu_for_each_leaf_node(rnp)
if (rcu_rnp_online_cpus(rnp))
rcu_spawn_one_boost_kthread(rnp);
}
#else /* #ifdef CONFIG_RCU_BOOST */
static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
__releases(rnp->lock)
{
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
}
static bool rcu_is_callbacks_kthread(void)
{
return false;
}
static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
{
}
static void rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
{
}
static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
{
}
static void __init rcu_spawn_boost_kthreads(void)
{
}
#endif /* #else #ifdef CONFIG_RCU_BOOST */
#if !defined(CONFIG_RCU_FAST_NO_HZ)
/*
* Check to see if any future non-offloaded RCU-related work will need
* to be done by the current CPU, even if none need be done immediately,
* returning 1 if so. This function is part of the RCU implementation;
* it is -not- an exported member of the RCU API.
*
* Because we not have RCU_FAST_NO_HZ, just check whether or not this
* CPU has RCU callbacks queued.
*/
int rcu_needs_cpu(u64 basemono, u64 *nextevt)
{
*nextevt = KTIME_MAX;
return !rcu_segcblist_empty(&this_cpu_ptr(&rcu_data)->cblist) &&
!rcu_rdp_is_offloaded(this_cpu_ptr(&rcu_data));
}
/*
* Because we do not have RCU_FAST_NO_HZ, don't bother cleaning up
* after it.
*/
static void rcu_cleanup_after_idle(void)
{
}
/*
* Do the idle-entry grace-period work, which, because CONFIG_RCU_FAST_NO_HZ=n,
* is nothing.
*/
static void rcu_prepare_for_idle(void)
{
}
#else /* #if !defined(CONFIG_RCU_FAST_NO_HZ) */
/*
* This code is invoked when a CPU goes idle, at which point we want
* to have the CPU do everything required for RCU so that it can enter
* the energy-efficient dyntick-idle mode.
*
* The following preprocessor symbol controls this:
*
* RCU_IDLE_GP_DELAY gives the number of jiffies that a CPU is permitted
* to sleep in dyntick-idle mode with RCU callbacks pending. This
* is sized to be roughly one RCU grace period. Those energy-efficiency
* benchmarkers who might otherwise be tempted to set this to a large
* number, be warned: Setting RCU_IDLE_GP_DELAY too high can hang your
* system. And if you are -that- concerned about energy efficiency,
* just power the system down and be done with it!
*
* The value below works well in practice. If future workloads require
* adjustment, they can be converted into kernel config parameters, though
* making the state machine smarter might be a better option.
*/
#define RCU_IDLE_GP_DELAY 4 /* Roughly one grace period. */
static int rcu_idle_gp_delay = RCU_IDLE_GP_DELAY;
module_param(rcu_idle_gp_delay, int, 0644);
/*
* Try to advance callbacks on the current CPU, but only if it has been
* awhile since the last time we did so. Afterwards, if there are any
* callbacks ready for immediate invocation, return true.
*/
static bool __maybe_unused rcu_try_advance_all_cbs(void)
{
bool cbs_ready = false;
struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
struct rcu_node *rnp;
/* Exit early if we advanced recently. */
if (jiffies == rdp->last_advance_all)
return false;
rdp->last_advance_all = jiffies;
rnp = rdp->mynode;
/*
* Don't bother checking unless a grace period has
* completed since we last checked and there are
* callbacks not yet ready to invoke.
*/
if ((rcu_seq_completed_gp(rdp->gp_seq,
rcu_seq_current(&rnp->gp_seq)) ||
unlikely(READ_ONCE(rdp->gpwrap))) &&
rcu_segcblist_pend_cbs(&rdp->cblist))
note_gp_changes(rdp);
if (rcu_segcblist_ready_cbs(&rdp->cblist))
cbs_ready = true;
return cbs_ready;
}
/*
* Allow the CPU to enter dyntick-idle mode unless it has callbacks ready
* to invoke. If the CPU has callbacks, try to advance them. Tell the
* caller about what to set the timeout.
*
* The caller must have disabled interrupts.
*/
int rcu_needs_cpu(u64 basemono, u64 *nextevt)
{
struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
unsigned long dj;
lockdep_assert_irqs_disabled();
/* If no non-offloaded callbacks, RCU doesn't need the CPU. */
if (rcu_segcblist_empty(&rdp->cblist) ||
rcu_rdp_is_offloaded(rdp)) {
*nextevt = KTIME_MAX;
return 0;
}
/* Attempt to advance callbacks. */
if (rcu_try_advance_all_cbs()) {
/* Some ready to invoke, so initiate later invocation. */
invoke_rcu_core();
return 1;
}
rdp->last_accelerate = jiffies;
/* Request timer and round. */
dj = round_up(rcu_idle_gp_delay + jiffies, rcu_idle_gp_delay) - jiffies;
*nextevt = basemono + dj * TICK_NSEC;
return 0;
}
/*
* Prepare a CPU for idle from an RCU perspective. The first major task is to
* sense whether nohz mode has been enabled or disabled via sysfs. The second
* major task is to accelerate (that is, assign grace-period numbers to) any
* recently arrived callbacks.
*
* The caller must have disabled interrupts.
*/
static void rcu_prepare_for_idle(void)
{
bool needwake;
struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
struct rcu_node *rnp;
int tne;
lockdep_assert_irqs_disabled();
if (rcu_rdp_is_offloaded(rdp))
return;
/* Handle nohz enablement switches conservatively. */
tne = READ_ONCE(tick_nohz_active);
if (tne != rdp->tick_nohz_enabled_snap) {
if (!rcu_segcblist_empty(&rdp->cblist))
invoke_rcu_core(); /* force nohz to see update. */
rdp->tick_nohz_enabled_snap = tne;
return;
}
if (!tne)
return;
/*
* If we have not yet accelerated this jiffy, accelerate all
* callbacks on this CPU.
*/
if (rdp->last_accelerate == jiffies)
return;
rdp->last_accelerate = jiffies;
if (rcu_segcblist_pend_cbs(&rdp->cblist)) {
rnp = rdp->mynode;
raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
needwake = rcu_accelerate_cbs(rnp, rdp);
raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */
if (needwake)
rcu_gp_kthread_wake();
}
}
/*
* Clean up for exit from idle. Attempt to advance callbacks based on
* any grace periods that elapsed while the CPU was idle, and if any
* callbacks are now ready to invoke, initiate invocation.
*/
static void rcu_cleanup_after_idle(void)
{
struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
lockdep_assert_irqs_disabled();
if (rcu_rdp_is_offloaded(rdp))
return;
if (rcu_try_advance_all_cbs())
invoke_rcu_core();
}
#endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */
/*
* Is this CPU a NO_HZ_FULL CPU that should ignore RCU so that the
* grace-period kthread will do force_quiescent_state() processing?
* The idea is to avoid waking up RCU core processing on such a
* CPU unless the grace period has extended for too long.
*
* This code relies on the fact that all NO_HZ_FULL CPUs are also
* CONFIG_RCU_NOCB_CPU CPUs.
*/
static bool rcu_nohz_full_cpu(void)
{
#ifdef CONFIG_NO_HZ_FULL
if (tick_nohz_full_cpu(smp_processor_id()) &&
(!rcu_gp_in_progress() ||
time_before(jiffies, READ_ONCE(rcu_state.gp_start) + HZ)))
return true;
#endif /* #ifdef CONFIG_NO_HZ_FULL */
return false;
}
/*
* Bind the RCU grace-period kthreads to the housekeeping CPU.
*/
static void rcu_bind_gp_kthread(void)
{
if (!tick_nohz_full_enabled())
return;
housekeeping_affine(current, HK_FLAG_RCU);
}
/* Record the current task on dyntick-idle entry. */
static void noinstr rcu_dynticks_task_enter(void)
{
#if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL)
WRITE_ONCE(current->rcu_tasks_idle_cpu, smp_processor_id());
#endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */
}
/* Record no current task on dyntick-idle exit. */
static void noinstr rcu_dynticks_task_exit(void)
{
#if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL)
WRITE_ONCE(current->rcu_tasks_idle_cpu, -1);
#endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */
}
/* Turn on heavyweight RCU tasks trace readers on idle/user entry. */
static void rcu_dynticks_task_trace_enter(void)
{
#ifdef CONFIG_TASKS_TRACE_RCU
if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
current->trc_reader_special.b.need_mb = true;
#endif /* #ifdef CONFIG_TASKS_TRACE_RCU */
}
/* Turn off heavyweight RCU tasks trace readers on idle/user exit. */
static void rcu_dynticks_task_trace_exit(void)
{
#ifdef CONFIG_TASKS_TRACE_RCU
if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
current->trc_reader_special.b.need_mb = false;
#endif /* #ifdef CONFIG_TASKS_TRACE_RCU */
}