qemu/util/aio-posix.c
Paolo Bonzini 5710a3e09f async: use explicit memory barriers
When using C11 atomics, non-seqcst reads and writes do not participate
in the total order of seqcst operations.  In util/async.c and util/aio-posix.c,
in particular, the pattern that we use

          write ctx->notify_me                 write bh->scheduled
          read bh->scheduled                   read ctx->notify_me
          if !bh->scheduled, sleep             if ctx->notify_me, notify

needs to use seqcst operations for both the write and the read.  In
general this is something that we do not want, because there can be
many sources that are polled in addition to bottom halves.  The
alternative is to place a seqcst memory barrier between the write
and the read.  This also comes with a disadvantage, in that the
memory barrier is implicit on strongly-ordered architectures and
it wastes a few dozen clock cycles.

Fortunately, ctx->notify_me is never written concurrently by two
threads, so we can assert that and relax the writes to ctx->notify_me.
The resulting solution works and performs well on both aarch64 and x86.

Note that the atomic_set/atomic_read combination is not an atomic
read-modify-write, and therefore it is even weaker than C11 ATOMIC_RELAXED;
on x86, ATOMIC_RELAXED compiles to a locked operation.

Analyzed-by: Ying Fang <fangying1@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Tested-by: Ying Fang <fangying1@huawei.com>
Message-Id: <20200407140746.8041-6-pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2020-04-09 16:17:14 +01:00

697 lines
20 KiB
C

/*
* QEMU aio implementation
*
* Copyright IBM, Corp. 2008
*
* Authors:
* Anthony Liguori <aliguori@us.ibm.com>
*
* This work is licensed under the terms of the GNU GPL, version 2. See
* the COPYING file in the top-level directory.
*
* Contributions after 2012-01-13 are licensed under the terms of the
* GNU GPL, version 2 or (at your option) any later version.
*/
#include "qemu/osdep.h"
#include "block/block.h"
#include "qemu/rcu.h"
#include "qemu/rcu_queue.h"
#include "qemu/sockets.h"
#include "qemu/cutils.h"
#include "trace.h"
#include "aio-posix.h"
/* Stop userspace polling on a handler if it isn't active for some time */
#define POLL_IDLE_INTERVAL_NS (7 * NANOSECONDS_PER_SECOND)
bool aio_poll_disabled(AioContext *ctx)
{
return atomic_read(&ctx->poll_disable_cnt);
}
void aio_add_ready_handler(AioHandlerList *ready_list,
AioHandler *node,
int revents)
{
QLIST_SAFE_REMOVE(node, node_ready); /* remove from nested parent's list */
node->pfd.revents = revents;
QLIST_INSERT_HEAD(ready_list, node, node_ready);
}
static AioHandler *find_aio_handler(AioContext *ctx, int fd)
{
AioHandler *node;
QLIST_FOREACH(node, &ctx->aio_handlers, node) {
if (node->pfd.fd == fd) {
if (!QLIST_IS_INSERTED(node, node_deleted)) {
return node;
}
}
}
return NULL;
}
static bool aio_remove_fd_handler(AioContext *ctx, AioHandler *node)
{
/* If the GSource is in the process of being destroyed then
* g_source_remove_poll() causes an assertion failure. Skip
* removal in that case, because glib cleans up its state during
* destruction anyway.
*/
if (!g_source_is_destroyed(&ctx->source)) {
g_source_remove_poll(&ctx->source, &node->pfd);
}
node->pfd.revents = 0;
/* If the fd monitor has already marked it deleted, leave it alone */
if (QLIST_IS_INSERTED(node, node_deleted)) {
return false;
}
/* If a read is in progress, just mark the node as deleted */
if (qemu_lockcnt_count(&ctx->list_lock)) {
QLIST_INSERT_HEAD_RCU(&ctx->deleted_aio_handlers, node, node_deleted);
return false;
}
/* Otherwise, delete it for real. We can't just mark it as
* deleted because deleted nodes are only cleaned up while
* no one is walking the handlers list.
*/
QLIST_SAFE_REMOVE(node, node_poll);
QLIST_REMOVE(node, node);
return true;
}
void aio_set_fd_handler(AioContext *ctx,
int fd,
bool is_external,
IOHandler *io_read,
IOHandler *io_write,
AioPollFn *io_poll,
void *opaque)
{
AioHandler *node;
AioHandler *new_node = NULL;
bool is_new = false;
bool deleted = false;
int poll_disable_change;
qemu_lockcnt_lock(&ctx->list_lock);
node = find_aio_handler(ctx, fd);
/* Are we deleting the fd handler? */
if (!io_read && !io_write && !io_poll) {
if (node == NULL) {
qemu_lockcnt_unlock(&ctx->list_lock);
return;
}
/* Clean events in order to unregister fd from the ctx epoll. */
node->pfd.events = 0;
poll_disable_change = -!node->io_poll;
} else {
poll_disable_change = !io_poll - (node && !node->io_poll);
if (node == NULL) {
is_new = true;
}
/* Alloc and insert if it's not already there */
new_node = g_new0(AioHandler, 1);
/* Update handler with latest information */
new_node->io_read = io_read;
new_node->io_write = io_write;
new_node->io_poll = io_poll;
new_node->opaque = opaque;
new_node->is_external = is_external;
if (is_new) {
new_node->pfd.fd = fd;
} else {
new_node->pfd = node->pfd;
}
g_source_add_poll(&ctx->source, &new_node->pfd);
new_node->pfd.events = (io_read ? G_IO_IN | G_IO_HUP | G_IO_ERR : 0);
new_node->pfd.events |= (io_write ? G_IO_OUT | G_IO_ERR : 0);
QLIST_INSERT_HEAD_RCU(&ctx->aio_handlers, new_node, node);
}
/* No need to order poll_disable_cnt writes against other updates;
* the counter is only used to avoid wasting time and latency on
* iterated polling when the system call will be ultimately necessary.
* Changing handlers is a rare event, and a little wasted polling until
* the aio_notify below is not an issue.
*/
atomic_set(&ctx->poll_disable_cnt,
atomic_read(&ctx->poll_disable_cnt) + poll_disable_change);
ctx->fdmon_ops->update(ctx, node, new_node);
if (node) {
deleted = aio_remove_fd_handler(ctx, node);
}
qemu_lockcnt_unlock(&ctx->list_lock);
aio_notify(ctx);
if (deleted) {
g_free(node);
}
}
void aio_set_fd_poll(AioContext *ctx, int fd,
IOHandler *io_poll_begin,
IOHandler *io_poll_end)
{
AioHandler *node = find_aio_handler(ctx, fd);
if (!node) {
return;
}
node->io_poll_begin = io_poll_begin;
node->io_poll_end = io_poll_end;
}
void aio_set_event_notifier(AioContext *ctx,
EventNotifier *notifier,
bool is_external,
EventNotifierHandler *io_read,
AioPollFn *io_poll)
{
aio_set_fd_handler(ctx, event_notifier_get_fd(notifier), is_external,
(IOHandler *)io_read, NULL, io_poll, notifier);
}
void aio_set_event_notifier_poll(AioContext *ctx,
EventNotifier *notifier,
EventNotifierHandler *io_poll_begin,
EventNotifierHandler *io_poll_end)
{
aio_set_fd_poll(ctx, event_notifier_get_fd(notifier),
(IOHandler *)io_poll_begin,
(IOHandler *)io_poll_end);
}
static bool poll_set_started(AioContext *ctx, bool started)
{
AioHandler *node;
bool progress = false;
if (started == ctx->poll_started) {
return false;
}
ctx->poll_started = started;
qemu_lockcnt_inc(&ctx->list_lock);
QLIST_FOREACH(node, &ctx->poll_aio_handlers, node_poll) {
IOHandler *fn;
if (QLIST_IS_INSERTED(node, node_deleted)) {
continue;
}
if (started) {
fn = node->io_poll_begin;
} else {
fn = node->io_poll_end;
}
if (fn) {
fn(node->opaque);
}
/* Poll one last time in case ->io_poll_end() raced with the event */
if (!started) {
progress = node->io_poll(node->opaque) || progress;
}
}
qemu_lockcnt_dec(&ctx->list_lock);
return progress;
}
bool aio_prepare(AioContext *ctx)
{
/* Poll mode cannot be used with glib's event loop, disable it. */
poll_set_started(ctx, false);
return false;
}
bool aio_pending(AioContext *ctx)
{
AioHandler *node;
bool result = false;
/*
* We have to walk very carefully in case aio_set_fd_handler is
* called while we're walking.
*/
qemu_lockcnt_inc(&ctx->list_lock);
QLIST_FOREACH_RCU(node, &ctx->aio_handlers, node) {
int revents;
revents = node->pfd.revents & node->pfd.events;
if (revents & (G_IO_IN | G_IO_HUP | G_IO_ERR) && node->io_read &&
aio_node_check(ctx, node->is_external)) {
result = true;
break;
}
if (revents & (G_IO_OUT | G_IO_ERR) && node->io_write &&
aio_node_check(ctx, node->is_external)) {
result = true;
break;
}
}
qemu_lockcnt_dec(&ctx->list_lock);
return result;
}
static void aio_free_deleted_handlers(AioContext *ctx)
{
AioHandler *node;
if (QLIST_EMPTY_RCU(&ctx->deleted_aio_handlers)) {
return;
}
if (!qemu_lockcnt_dec_if_lock(&ctx->list_lock)) {
return; /* we are nested, let the parent do the freeing */
}
while ((node = QLIST_FIRST_RCU(&ctx->deleted_aio_handlers))) {
QLIST_REMOVE(node, node);
QLIST_REMOVE(node, node_deleted);
QLIST_SAFE_REMOVE(node, node_poll);
g_free(node);
}
qemu_lockcnt_inc_and_unlock(&ctx->list_lock);
}
static bool aio_dispatch_handler(AioContext *ctx, AioHandler *node)
{
bool progress = false;
int revents;
revents = node->pfd.revents & node->pfd.events;
node->pfd.revents = 0;
/*
* Start polling AioHandlers when they become ready because activity is
* likely to continue. Note that starvation is theoretically possible when
* fdmon_supports_polling(), but only until the fd fires for the first
* time.
*/
if (!QLIST_IS_INSERTED(node, node_deleted) &&
!QLIST_IS_INSERTED(node, node_poll) &&
node->io_poll) {
trace_poll_add(ctx, node, node->pfd.fd, revents);
if (ctx->poll_started && node->io_poll_begin) {
node->io_poll_begin(node->opaque);
}
QLIST_INSERT_HEAD(&ctx->poll_aio_handlers, node, node_poll);
}
if (!QLIST_IS_INSERTED(node, node_deleted) &&
(revents & (G_IO_IN | G_IO_HUP | G_IO_ERR)) &&
aio_node_check(ctx, node->is_external) &&
node->io_read) {
node->io_read(node->opaque);
/* aio_notify() does not count as progress */
if (node->opaque != &ctx->notifier) {
progress = true;
}
}
if (!QLIST_IS_INSERTED(node, node_deleted) &&
(revents & (G_IO_OUT | G_IO_ERR)) &&
aio_node_check(ctx, node->is_external) &&
node->io_write) {
node->io_write(node->opaque);
progress = true;
}
return progress;
}
/*
* If we have a list of ready handlers then this is more efficient than
* scanning all handlers with aio_dispatch_handlers().
*/
static bool aio_dispatch_ready_handlers(AioContext *ctx,
AioHandlerList *ready_list)
{
bool progress = false;
AioHandler *node;
while ((node = QLIST_FIRST(ready_list))) {
QLIST_REMOVE(node, node_ready);
progress = aio_dispatch_handler(ctx, node) || progress;
}
return progress;
}
/* Slower than aio_dispatch_ready_handlers() but only used via glib */
static bool aio_dispatch_handlers(AioContext *ctx)
{
AioHandler *node, *tmp;
bool progress = false;
QLIST_FOREACH_SAFE_RCU(node, &ctx->aio_handlers, node, tmp) {
progress = aio_dispatch_handler(ctx, node) || progress;
}
return progress;
}
void aio_dispatch(AioContext *ctx)
{
qemu_lockcnt_inc(&ctx->list_lock);
aio_bh_poll(ctx);
aio_dispatch_handlers(ctx);
aio_free_deleted_handlers(ctx);
qemu_lockcnt_dec(&ctx->list_lock);
timerlistgroup_run_timers(&ctx->tlg);
}
static bool run_poll_handlers_once(AioContext *ctx,
int64_t now,
int64_t *timeout)
{
bool progress = false;
AioHandler *node;
AioHandler *tmp;
QLIST_FOREACH_SAFE(node, &ctx->poll_aio_handlers, node_poll, tmp) {
if (aio_node_check(ctx, node->is_external) &&
node->io_poll(node->opaque)) {
node->poll_idle_timeout = now + POLL_IDLE_INTERVAL_NS;
/*
* Polling was successful, exit try_poll_mode immediately
* to adjust the next polling time.
*/
*timeout = 0;
if (node->opaque != &ctx->notifier) {
progress = true;
}
}
/* Caller handles freeing deleted nodes. Don't do it here. */
}
return progress;
}
static bool fdmon_supports_polling(AioContext *ctx)
{
return ctx->fdmon_ops->need_wait != aio_poll_disabled;
}
static bool remove_idle_poll_handlers(AioContext *ctx, int64_t now)
{
AioHandler *node;
AioHandler *tmp;
bool progress = false;
/*
* File descriptor monitoring implementations without userspace polling
* support suffer from starvation when a subset of handlers is polled
* because fds will not be processed in a timely fashion. Don't remove
* idle poll handlers.
*/
if (!fdmon_supports_polling(ctx)) {
return false;
}
QLIST_FOREACH_SAFE(node, &ctx->poll_aio_handlers, node_poll, tmp) {
if (node->poll_idle_timeout == 0LL) {
node->poll_idle_timeout = now + POLL_IDLE_INTERVAL_NS;
} else if (now >= node->poll_idle_timeout) {
trace_poll_remove(ctx, node, node->pfd.fd);
node->poll_idle_timeout = 0LL;
QLIST_SAFE_REMOVE(node, node_poll);
if (ctx->poll_started && node->io_poll_end) {
node->io_poll_end(node->opaque);
/*
* Final poll in case ->io_poll_end() races with an event.
* Nevermind about re-adding the handler in the rare case where
* this causes progress.
*/
progress = node->io_poll(node->opaque) || progress;
}
}
}
return progress;
}
/* run_poll_handlers:
* @ctx: the AioContext
* @max_ns: maximum time to poll for, in nanoseconds
*
* Polls for a given time.
*
* Note that ctx->notify_me must be non-zero so this function can detect
* aio_notify().
*
* Note that the caller must have incremented ctx->list_lock.
*
* Returns: true if progress was made, false otherwise
*/
static bool run_poll_handlers(AioContext *ctx, int64_t max_ns, int64_t *timeout)
{
bool progress;
int64_t start_time, elapsed_time;
assert(ctx->notify_me);
assert(qemu_lockcnt_count(&ctx->list_lock) > 0);
trace_run_poll_handlers_begin(ctx, max_ns, *timeout);
/*
* Optimization: ->io_poll() handlers often contain RCU read critical
* sections and we therefore see many rcu_read_lock() -> rcu_read_unlock()
* -> rcu_read_lock() -> ... sequences with expensive memory
* synchronization primitives. Make the entire polling loop an RCU
* critical section because nested rcu_read_lock()/rcu_read_unlock() calls
* are cheap.
*/
RCU_READ_LOCK_GUARD();
start_time = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
do {
progress = run_poll_handlers_once(ctx, start_time, timeout);
elapsed_time = qemu_clock_get_ns(QEMU_CLOCK_REALTIME) - start_time;
max_ns = qemu_soonest_timeout(*timeout, max_ns);
assert(!(max_ns && progress));
} while (elapsed_time < max_ns && !ctx->fdmon_ops->need_wait(ctx));
if (remove_idle_poll_handlers(ctx, start_time + elapsed_time)) {
*timeout = 0;
progress = true;
}
/* If time has passed with no successful polling, adjust *timeout to
* keep the same ending time.
*/
if (*timeout != -1) {
*timeout -= MIN(*timeout, elapsed_time);
}
trace_run_poll_handlers_end(ctx, progress, *timeout);
return progress;
}
/* try_poll_mode:
* @ctx: the AioContext
* @timeout: timeout for blocking wait, computed by the caller and updated if
* polling succeeds.
*
* ctx->notify_me must be non-zero so this function can detect aio_notify().
*
* Note that the caller must have incremented ctx->list_lock.
*
* Returns: true if progress was made, false otherwise
*/
static bool try_poll_mode(AioContext *ctx, int64_t *timeout)
{
int64_t max_ns;
if (QLIST_EMPTY_RCU(&ctx->poll_aio_handlers)) {
return false;
}
max_ns = qemu_soonest_timeout(*timeout, ctx->poll_ns);
if (max_ns && !ctx->fdmon_ops->need_wait(ctx)) {
poll_set_started(ctx, true);
if (run_poll_handlers(ctx, max_ns, timeout)) {
return true;
}
}
if (poll_set_started(ctx, false)) {
*timeout = 0;
return true;
}
return false;
}
bool aio_poll(AioContext *ctx, bool blocking)
{
AioHandlerList ready_list = QLIST_HEAD_INITIALIZER(ready_list);
int ret = 0;
bool progress;
int64_t timeout;
int64_t start = 0;
/*
* There cannot be two concurrent aio_poll calls for the same AioContext (or
* an aio_poll concurrent with a GSource prepare/check/dispatch callback).
* We rely on this below to avoid slow locked accesses to ctx->notify_me.
*/
assert(in_aio_context_home_thread(ctx));
/* aio_notify can avoid the expensive event_notifier_set if
* everything (file descriptors, bottom halves, timers) will
* be re-evaluated before the next blocking poll(). This is
* already true when aio_poll is called with blocking == false;
* if blocking == true, it is only true after poll() returns,
* so disable the optimization now.
*/
if (blocking) {
atomic_set(&ctx->notify_me, atomic_read(&ctx->notify_me) + 2);
/*
* Write ctx->notify_me before computing the timeout
* (reading bottom half flags, etc.). Pairs with
* smp_mb in aio_notify().
*/
smp_mb();
}
qemu_lockcnt_inc(&ctx->list_lock);
if (ctx->poll_max_ns) {
start = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
}
timeout = blocking ? aio_compute_timeout(ctx) : 0;
progress = try_poll_mode(ctx, &timeout);
assert(!(timeout && progress));
/* If polling is allowed, non-blocking aio_poll does not need the
* system call---a single round of run_poll_handlers_once suffices.
*/
if (timeout || ctx->fdmon_ops->need_wait(ctx)) {
ret = ctx->fdmon_ops->wait(ctx, &ready_list, timeout);
}
if (blocking) {
/* Finish the poll before clearing the flag. */
atomic_store_release(&ctx->notify_me, atomic_read(&ctx->notify_me) - 2);
aio_notify_accept(ctx);
}
/* Adjust polling time */
if (ctx->poll_max_ns) {
int64_t block_ns = qemu_clock_get_ns(QEMU_CLOCK_REALTIME) - start;
if (block_ns <= ctx->poll_ns) {
/* This is the sweet spot, no adjustment needed */
} else if (block_ns > ctx->poll_max_ns) {
/* We'd have to poll for too long, poll less */
int64_t old = ctx->poll_ns;
if (ctx->poll_shrink) {
ctx->poll_ns /= ctx->poll_shrink;
} else {
ctx->poll_ns = 0;
}
trace_poll_shrink(ctx, old, ctx->poll_ns);
} else if (ctx->poll_ns < ctx->poll_max_ns &&
block_ns < ctx->poll_max_ns) {
/* There is room to grow, poll longer */
int64_t old = ctx->poll_ns;
int64_t grow = ctx->poll_grow;
if (grow == 0) {
grow = 2;
}
if (ctx->poll_ns) {
ctx->poll_ns *= grow;
} else {
ctx->poll_ns = 4000; /* start polling at 4 microseconds */
}
if (ctx->poll_ns > ctx->poll_max_ns) {
ctx->poll_ns = ctx->poll_max_ns;
}
trace_poll_grow(ctx, old, ctx->poll_ns);
}
}
progress |= aio_bh_poll(ctx);
if (ret > 0) {
progress |= aio_dispatch_ready_handlers(ctx, &ready_list);
}
aio_free_deleted_handlers(ctx);
qemu_lockcnt_dec(&ctx->list_lock);
progress |= timerlistgroup_run_timers(&ctx->tlg);
return progress;
}
void aio_context_setup(AioContext *ctx)
{
ctx->fdmon_ops = &fdmon_poll_ops;
ctx->epollfd = -1;
/* Use the fastest fd monitoring implementation if available */
if (fdmon_io_uring_setup(ctx)) {
return;
}
fdmon_epoll_setup(ctx);
}
void aio_context_destroy(AioContext *ctx)
{
fdmon_io_uring_destroy(ctx);
fdmon_epoll_disable(ctx);
}
void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
int64_t grow, int64_t shrink, Error **errp)
{
/* No thread synchronization here, it doesn't matter if an incorrect value
* is used once.
*/
ctx->poll_max_ns = max_ns;
ctx->poll_ns = 0;
ctx->poll_grow = grow;
ctx->poll_shrink = shrink;
aio_notify(ctx);
}