Commit e554413bd2 for qemu.org
commit e554413bd2b4d6e1f31eebfb499bce5238b91646
Author: Denis V. Lunev <den@openvz.org>
Date: Wed Jun 10 13:58:50 2026 +0200
coroutine: fix lost wakeup in qemu_co_sleep_wake()
cache_clean_timer_del_and_wait() cancels the cache-cleaner coroutine
by setting s->cache_clean_interval = 0 and calling qemu_co_sleep_wake()
to cut short its qemu_co_sleep_ns_wakeable(). qemu_co_sleep_wake() is
fire-and-forget: it reads w->to_wake and silently returns when it is
NULL. A sleeper that is between two iterations -- has just released
s->lock but has not yet set w->to_wake inside qemu_co_sleep() -- loses
the wake:
iothread0 timer coroutine main thread (qcow2 close)
------------------------- -------------------------
while-body (holding s->lock):
read interval = 600
wait_ns = 600 * NS
release s->lock
take s->lock
interval = 0
qemu_co_sleep_wake(w):
w->to_wake == NULL -> skip
return
qemu_co_queue_wait(exit, s->lock):
release s->lock
yield
qemu_co_sleep_ns_wakeable:
aio_timer_init(+600 s)
qemu_co_sleep:
cas scheduled NULL -> "qsns"
w->to_wake = co
yield [sleeps 600 s]
cache_clean_timer_del_and_wait() then blocks on cache_clean_timer_exit
until the original 600 s expiry fires, and qcow2_close() holds BQL the
whole time so the VM stalls behind it.
block_copy_kick() has the same shape. Fix the primitive once instead
of working around it in each caller.
Use a tri-state for QemuCoSleep::to_wake:
NULL - idle
co - sleeper parked
PENDING - wake delivered, no sleeper yet (sticky)
qemu_co_sleep_wake() xchgs PENDING into to_wake: a real sleeper is
woken, NULL/PENDING is left untouched so the wake stays sticky.
qemu_co_sleep() cmpxchg-publishes itself as the sleeper; if a wake
was delivered before it got there or races the publish, the cmpxchg
observes PENDING and returns without yielding. On normal resume
qemu_co_sleep() clears the PENDING the waker left behind so the next
sleep starts clean.
A double-fire (real wake plus timer callback) is harmless: the first
xchg returns the coroutine and wakes it; the second returns PENDING
and is a no-op. Cancellation latency through qemu_co_sleep_wake() is
now bounded by aio_co_wake() rather than by the sleep duration.
Fixes: f86dde9a15 ("qcow2: Fix cache_clean_timer")
Signed-off-by: Denis V. Lunev <den@openvz.org>
Cc: Hanna Czenczek <hreitz@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20260610115850.2410566-2-den@openvz.org>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/include/qemu/coroutine.h b/include/qemu/coroutine.h
index e545bbf620..1c31de60f9 100644
--- a/include/qemu/coroutine.h
+++ b/include/qemu/coroutine.h
@@ -260,10 +260,19 @@ int coroutine_fn qemu_co_timeout(CoroutineEntry *entry, void *opaque,
uint64_t timeout_ns, CleanupFunc clean);
/**
- * Wake a coroutine if it is sleeping in qemu_co_sleep_ns. The timer will be
- * deleted. @sleep_state must be the variable whose address was given to
- * qemu_co_sleep_ns() and should be checked to be non-NULL before calling
- * qemu_co_sleep_wake().
+ * Wake a coroutine sleeping in qemu_co_sleep() or qemu_co_sleep_ns_wakeable().
+ * The timer set up by the latter is deleted on wakeup.
+ *
+ * The wake is sticky: if no sleeper is parked on @w at the time of the call,
+ * the wake is recorded on @w and consumed by the next qemu_co_sleep() on the
+ * same @w, which then returns without yielding. This closes the lost-wakeup
+ * window between two sleeps and is the documented behavior callers should
+ * rely on -- e.g. a cancellation signal raised between iterations of a
+ * sleep/work loop will shorten the next sleep instead of being dropped.
+ *
+ * The state persists until consumed: if no further qemu_co_sleep() is ever
+ * called on @w, the pending wake is harmlessly discarded when @w goes away.
+ * Multiple wakes coalesce -- the next sleep consumes at most one.
*/
void qemu_co_sleep_wake(QemuCoSleep *w);
diff --git a/tests/unit/test-coroutine.c b/tests/unit/test-coroutine.c
index 49d4d9b251..aa1f719b08 100644
--- a/tests/unit/test-coroutine.c
+++ b/tests/unit/test-coroutine.c
@@ -421,6 +421,57 @@ static void test_co_rwlock_downgrade(void)
g_assert(c1_done);
}
+/*
+ * Check that a wake delivered before the sleeper parks is not lost.
+ *
+ * qemu_co_sleep_wake() is fire-and-forget: a caller cancelling a
+ * sleep/work loop may call it in the window after the sleeper has
+ * decided to sleep but before it has published itself inside
+ * qemu_co_sleep(). The wake must be sticky and shorten the next sleep
+ * rather than being dropped (which would block until the full sleep
+ * duration expired).
+ *
+ * No threads, timers or AioContext are needed: coroutines are
+ * cooperative, so ordering the wake before the sleep deterministically
+ * reproduces the state the racing waker would otherwise produce.
+ */
+
+typedef struct {
+ QemuCoSleep w;
+ bool completed;
+} CoSleepWakeData;
+
+static void coroutine_fn co_sleep_wake_entry(void *opaque)
+{
+ CoSleepWakeData *d = opaque;
+
+ /*
+ * The wake was already delivered before we got here. qemu_co_sleep()
+ * must consume it and return without yielding.
+ */
+ qemu_co_sleep(&d->w);
+ d->completed = true;
+}
+
+static void test_co_sleep_wake_before_sleep(void)
+{
+ CoSleepWakeData d = { .w = { 0 }, .completed = false };
+ Coroutine *co = qemu_coroutine_create(co_sleep_wake_entry, &d);
+
+ /* Waker runs first, while no sleeper is parked on w. */
+ qemu_co_sleep_wake(&d.w);
+
+ /*
+ * Entering runs qemu_co_sleep(), which consumes the pending wake and
+ * returns without yielding, so the coroutine runs straight to
+ * completion in this single enter. With the pre-fix primitive the wake
+ * is dropped, qemu_co_sleep() parks, and completed stays false.
+ */
+ qemu_coroutine_enter(co);
+
+ g_assert(d.completed);
+}
+
/*
* Check that creation, enter, and return work
*/
@@ -660,6 +711,8 @@ int main(int argc, char **argv)
g_test_add_func("/locking/co-mutex/lockable", test_co_mutex_lockable);
g_test_add_func("/locking/co-rwlock/upgrade", test_co_rwlock_upgrade);
g_test_add_func("/locking/co-rwlock/downgrade", test_co_rwlock_downgrade);
+ g_test_add_func("/locking/co-sleep/wake-before-sleep",
+ test_co_sleep_wake_before_sleep);
if (g_test_perf()) {
g_test_add_func("/perf/lifecycle", perf_lifecycle);
g_test_add_func("/perf/nesting", perf_nesting);
diff --git a/util/qemu-coroutine-sleep.c b/util/qemu-coroutine-sleep.c
index edef117284..19ded0b6fd 100644
--- a/util/qemu-coroutine-sleep.c
+++ b/util/qemu-coroutine-sleep.c
@@ -18,20 +18,29 @@
static const char *qemu_co_sleep_ns__scheduled = "qemu_co_sleep_ns";
+/*
+ * Sentinel stored in QemuCoSleep::to_wake by qemu_co_sleep_wake() when no
+ * sleeper has parked yet. The next qemu_co_sleep() consumes it and returns
+ * without yielding, so a wake that races the arming of a sleep is never
+ * lost.
+ */
+#define QEMU_CO_SLEEP_PENDING ((Coroutine *)(uintptr_t)1)
+
void qemu_co_sleep_wake(QemuCoSleep *w)
{
Coroutine *co;
- co = w->to_wake;
- w->to_wake = NULL;
- if (co) {
- /* Write of schedule protected by barrier write in aio_co_schedule */
- const char *scheduled = qatomic_cmpxchg(&co->scheduled,
- qemu_co_sleep_ns__scheduled, NULL);
-
- assert(scheduled == qemu_co_sleep_ns__scheduled);
- aio_co_wake(co);
+ co = qatomic_xchg(&w->to_wake, QEMU_CO_SLEEP_PENDING);
+ if (co == NULL || co == QEMU_CO_SLEEP_PENDING) {
+ /* No sleeper, or a wake is already pending. */
+ return;
}
+
+ /* Write of scheduled protected by barrier write in aio_co_schedule */
+ const char *scheduled = qatomic_cmpxchg(&co->scheduled,
+ qemu_co_sleep_ns__scheduled, NULL);
+ assert(scheduled == qemu_co_sleep_ns__scheduled);
+ aio_co_wake(co);
}
static void co_sleep_cb(void *opaque)
@@ -43,6 +52,7 @@ static void co_sleep_cb(void *opaque)
void coroutine_fn qemu_co_sleep(QemuCoSleep *w)
{
Coroutine *co = qemu_coroutine_self();
+ Coroutine *prev;
const char *scheduled = qatomic_cmpxchg(&co->scheduled, NULL,
qemu_co_sleep_ns__scheduled);
@@ -53,11 +63,23 @@ void coroutine_fn qemu_co_sleep(QemuCoSleep *w)
abort();
}
- w->to_wake = co;
+ /*
+ * Publish ourselves as the sleeper. A wake delivered before we got here,
+ * or one racing this publish, leaves QEMU_CO_SLEEP_PENDING in to_wake;
+ * the cmpxchg then fails and we consume the wake without yielding.
+ */
+ prev = qatomic_cmpxchg(&w->to_wake, NULL, co);
+ if (prev == QEMU_CO_SLEEP_PENDING) {
+ qatomic_set(&w->to_wake, NULL);
+ qatomic_set(&co->scheduled, NULL);
+ return;
+ }
+ assert(prev == NULL);
+
qemu_coroutine_yield();
- /* w->to_wake is cleared before resuming this coroutine. */
- assert(w->to_wake == NULL);
+ /* The waker left QEMU_CO_SLEEP_PENDING; clear it for the next sleep. */
+ qatomic_set(&w->to_wake, NULL);
}
void coroutine_fn qemu_co_sleep_ns_wakeable(QemuCoSleep *w,
@@ -70,9 +92,10 @@ void coroutine_fn qemu_co_sleep_ns_wakeable(QemuCoSleep *w,
timer_mod(&ts, qemu_clock_get_ns(type) + ns);
/*
- * The timer will fire in the current AiOContext, so the callback
- * must happen after qemu_co_sleep yields and there is no race
- * between timer_mod and qemu_co_sleep.
+ * A wake racing with the arming of the sleep -- including the timer
+ * we just armed firing in another AioContext before qemu_co_sleep()
+ * publishes itself -- is captured by the sticky PENDING state in
+ * qemu_co_sleep_wake() and consumed here without yielding.
*/
qemu_co_sleep(w);
timer_del(&ts);