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() consumes a sticky PENDING on entry; otherwise it
cmpxchg-publishes itself, and a wake racing in between is caught
because 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 <[email protected]>
Cc: Hanna Czenczek <[email protected]>
Cc: Kevin Wolf <[email protected]>
---
include/qemu/coroutine.h | 17 ++++++++---
util/qemu-coroutine-sleep.c | 60 +++++++++++++++++++++++++++----------
2 files changed, 58 insertions(+), 19 deletions(-)
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/util/qemu-coroutine-sleep.c b/util/qemu-coroutine-sleep.c
index edef117284..a5734a8eb7 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 +52,14 @@ static void co_sleep_cb(void *opaque)
void coroutine_fn qemu_co_sleep(QemuCoSleep *w)
{
Coroutine *co = qemu_coroutine_self();
+ Coroutine *prev;
+
+ /* Consume an already-delivered wake without yielding. */
+ prev = qatomic_cmpxchg(&w->to_wake, QEMU_CO_SLEEP_PENDING, NULL);
+ if (prev == QEMU_CO_SLEEP_PENDING) {
+ return;
+ }
+ assert(prev == NULL);
const char *scheduled = qatomic_cmpxchg(&co->scheduled, NULL,
qemu_co_sleep_ns__scheduled);
@@ -53,11 +70,23 @@ void coroutine_fn qemu_co_sleep(QemuCoSleep *w)
abort();
}
- w->to_wake = co;
+ /*
+ * Publish ourselves as the sleeper. If a wake raced in between the
+ * fast path above and now, the cmpxchg observes QEMU_CO_SLEEP_PENDING
+ * and we consume it 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 +99,10 @@ 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);
--
2.51.0
