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Lunev" Cc: qemu-devel@nongnu.org, qemu-block@nongnu.org, qemu-stable@nongnu.org, hreitz@redhat.com, stefanha@redhat.com Subject: Re: [PATCH v2 1/1] coroutine: fix lost wakeup in qemu_co_sleep_wake() Message-ID: References: <20260520193849.467936-1-den@openvz.org> <20260520193849.467936-2-den@openvz.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260520193849.467936-2-den@openvz.org> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 Received-SPF: pass client-ip=170.10.133.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: 8 X-Spam_score: 0.8 X-Spam_bar: / X-Spam_report: (0.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.445, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_SBL_CSS=3.335, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Am 20.05.2026 um 21:38 hat Denis V. Lunev geschrieben: > 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 > Cc: Hanna Czenczek > Cc: Kevin Wolf > --- > 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); I'm wondering if this check serves a purpose when... > 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) { ...we essentially have to repeat it here anyway. Couldn't we always just use this second check? > + 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); Overall this looks good to me. Some unit tests would be nice, though. Kevin