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(p200300cfd7171f4cb75799638006395e.dip0.t-ipconnect.de. [2003:cf:d717:1f4c:b757:9963:8006:395e]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-429c13e1c9esm2632417f8f.22.2025.10.31.02.29.08 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 31 Oct 2025 02:29:09 -0700 (PDT) Message-ID: <0d98f477-722c-4023-9b28-54d8faffff66@redhat.com> Date: Fri, 31 Oct 2025 10:29:07 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 10/16] qcow2: Fix cache_clean_timer To: Kevin Wolf Cc: qemu-block@nongnu.org, qemu-devel@nongnu.org, Stefan Hajnoczi , Paolo Bonzini , "Richard W . M . Jones" , Ilya Dryomov , Peter Lieven , =?UTF-8?Q?Philippe_Mathieu-Daud=C3=A9?= , =?UTF-8?Q?Alex_Benn=C3=A9e?= , Fam Zheng , Ronnie Sahlberg References: <20251028163343.116249-1-hreitz@redhat.com> <20251028163343.116249-11-hreitz@redhat.com> Content-Language: en-US From: Hanna Czenczek In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=hreitz@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, 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_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_PASS=-0.001, T_SPF_TEMPERROR=0.01 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: 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 On 29.10.25 21:23, Kevin Wolf wrote: > Am 28.10.2025 um 17:33 hat Hanna Czenczek geschrieben: >> The cache-cleaner runs as a timer CB in the BDS AioContext. With >> multiqueue, it can run concurrently to I/O requests, and because it does >> not take any lock, this can break concurrent cache accesses, corrupting >> the image. While the chances of this happening are low, it can be >> reproduced e.g. by modifying the code to schedule the timer CB every >> 5 ms (instead of at most once per second) and modifying the last (inner) >> while loop of qcow2_cache_clean_unused() like so: >> >> while (i < c->size && can_clean_entry(c, i)) { >> for (int j = 0; j < 1000 && can_clean_entry(c, i); j++) { >> usleep(100); >> } >> c->entries[i].offset = 0; >> c->entries[i].lru_counter = 0; >> i++; >> to_clean++; >> } >> >> i.e. making it wait on purpose for the point in time where the cache is >> in use by something else. >> >> The solution chosen for this in this patch is not the best solution, I >> hope, but I admittedly can’t come up with anything strictly better. >> >> We can protect from concurrent cache accesses either by taking the >> existing s->lock, or we introduce a new (non-coroutine) mutex >> specifically for cache accesses. I would prefer to avoid the latter so >> as not to introduce additional (very slight) overhead. > In theory, the old plan was that eventually qcow2 would use fine grained > locks instead of the single s->lock, and having a separate cache lock > would be a step towards it. But if we never actually make use of it to > hold s->lock for a shorter time, that's not really a good argument. I'm > not sure if that's ever going to happen unless for a rewrite in Rust or > something. > > I never tried to measure specifically if lock contention is a problem > with high queue depth and random I/O on a huge disk. Intuitively, > holding s->lock while doing I/O for loading entries into the cache can't > be really good. > > Anyway, I went a bit on a tangent there... > >> Using s->lock, which is a coroutine mutex, however means that we need to >> take it in a coroutine, so the timer CB must enter such a coroutine. As >> a result, descheduling the timer is no longer a guarantee that the >> cache-cleaner will not run, because it may now be yielding in >> qemu_co_mutex_lock(). > I think creating a coroutine in cache_clean_timer_cb() is the wrong > approach. Instead, cache_clean_timer_init() could create a coroutine > and its implementation could be something like this: > > while (!s->cache_clean_timer_stopping) { > qemu_co_sleep_ns_wakeable(&s->cache_clean_timer_wake, > QEMU_CLOCK_VIRTUAL, > s->cache_clean_interval * NANOSECONDS_PER_SECOND); > > qemu_co_mutex_lock(&s->lock); > qcow2_cache_clean_unused(s->l2_table_cache); > qcow2_cache_clean_unused(s->refcount_block_cache); > qemu_co_mutex_unlock(&s->lock); > } > s->cache_clean_timer_stopping = false; Ah, that’s nicer.  I think we can replace the flag by checking s->cache_clean_interval > 0 and adding a CoQueue to wake up waiting coroutines. >> (Note even now this was only guaranteed for cache_clean_timer_del() >> callers that run in the BDS (the timer’s) AioContext. For callers >> running in the main context, the problem may have already existed, >> though maybe the BQL prevents timers from running in other contexts, I’m >> not sure.) >> >> Polling to await the timer to actually settle seems very complicated for >> something that’s rather a minor problem, but I can’t come up with any >> better solution that doesn’t again just overlook potential problems. >> >> (One cleaner idea may be to have a generic way to have timers run >> coroutines, and to await those when descheduling the timer. But while >> cleaner, it would also be more complicated, and I don’t think worth it >> at this point.) >> >> (Not Cc-ing qemu-stable, as the issue is quite unlikely to be hit, and >> I’m not too fond of this solution.) >> >> Signed-off-by: Hanna Czenczek >> --- >> block/qcow2.h | 1 + >> block/qcow2.c | 90 ++++++++++++++++++++++++++++++++++++++++++++------- >> 2 files changed, 79 insertions(+), 12 deletions(-) >> @@ -867,6 +893,39 @@ static void cache_clean_timer_del(BlockDriverState *bs) >> } >> } >> >> +/* >> + * Delete the cache clean timer and await any yet running instance. >> + * Must be called from the main or BDS AioContext, holding s->lock. >> + */ >> +static void coroutine_fn >> +cache_clean_timer_locked_co_del_and_wait(BlockDriverState *bs) >> +{ >> + BDRVQcow2State *s = bs->opaque; >> + IO_OR_GS_CODE(); >> + cache_clean_timer_del(bs); >> + if (qatomic_read(&s->cache_clean_running)) { >> + qemu_co_mutex_unlock(&s->lock); >> + qatomic_set(&s->cache_clean_polling, true); >> + BDRV_POLL_WHILE(bs, qatomic_read(&s->cache_clean_running)); > Polling in a coroutine_fn is verboten. > > If we do need this function, I think it would be a yield here and a wake > on the other side. I think we might be able to get around it if we move > the call from qcow2_do_open() into qcow2_open() (i.e. outside the > coroutine). A bit ugly, so your choice. We can let a CoQueue do the waking, no? >> + qemu_co_mutex_lock(&s->lock); >> + } >> +} >> + >> +/* >> + * Delete the cache clean timer and await any yet running instance. >> + * Must be called from the main or BDS AioContext without s->lock held. >> + */ >> +static void cache_clean_timer_del_and_wait(BlockDriverState *bs) >> +{ >> + BDRVQcow2State *s = bs->opaque; >> + IO_OR_GS_CODE(); >> + cache_clean_timer_del(bs); >> + if (qatomic_read(&s->cache_clean_running)) { >> + qatomic_set(&s->cache_clean_polling, true); >> + BDRV_POLL_WHILE(bs, qatomic_read(&s->cache_clean_running)); >> + } >> +} >> + >> static void qcow2_detach_aio_context(BlockDriverState *bs) >> { >> cache_clean_timer_del(bs); >> @@ -1214,12 +1273,20 @@ fail: >> return ret; >> } >> >> +/* s_locked specifies whether s->lock is held or not */ >> static void qcow2_update_options_commit(BlockDriverState *bs, >> - Qcow2ReopenState *r) >> + Qcow2ReopenState *r, >> + bool s_locked) >> { >> BDRVQcow2State *s = bs->opaque; >> int i; >> >> + if (s_locked) { >> + cache_clean_timer_locked_co_del_and_wait(bs); >> + } else { >> + cache_clean_timer_del_and_wait(bs); >> + } >> + >> if (s->l2_table_cache) { >> qcow2_cache_destroy(s->l2_table_cache); >> } >> @@ -1228,6 +1295,10 @@ static void qcow2_update_options_commit(BlockDriverState *bs, >> } >> s->l2_table_cache = r->l2_table_cache; >> s->refcount_block_cache = r->refcount_block_cache; >> + >> + s->cache_clean_interval = r->cache_clean_interval; >> + cache_clean_timer_init(bs, bdrv_get_aio_context(bs)); >> + >> s->l2_slice_size = r->l2_slice_size; >> >> s->overlap_check = r->overlap_check; >> @@ -1239,12 +1310,6 @@ static void qcow2_update_options_commit(BlockDriverState *bs, >> >> s->discard_no_unref = r->discard_no_unref; >> >> - if (s->cache_clean_interval != r->cache_clean_interval) { >> - cache_clean_timer_del(bs); >> - s->cache_clean_interval = r->cache_clean_interval; >> - cache_clean_timer_init(bs, bdrv_get_aio_context(bs)); >> - } >> - > I think the del/init pair here won't be necessary any more after > switching to the background coroutine. It will just start using the new > value of s->cache_clean_interval the next time it sleeps. One problem is that if we don’t lock s->lock, the coroutine can read s->l2_table_cache and s->refcount_block_cache while they’re invalid, which is why I moved the deletion above.  We also still need to delete if the interval is set to 0 (or special-case that in the coroutine to wait forever). We could run all of this in a coroutine so we can lock s->lock, or we have to force-stop the timer/coroutine at the start.  Maybe running it in a coroutine is better… Hanna