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(p200300cfd7171f76905c29a8188b2aad.dip0.t-ipconnect.de. [2003:cf:d717:1f76:905c:29a8:188b:2aad]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4779a2892c8sm183481855e9.1.2025.11.18.03.01.34 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 18 Nov 2025 03:01:34 -0800 (PST) Message-ID: <7a3dc3f2-cded-4e2f-a914-938acd52576f@redhat.com> Date: Tue, 18 Nov 2025 12:01:31 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 12/19] 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: <20251110154854.151484-1-hreitz@redhat.com> <20251110154854.151484-13-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_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 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 17.11.25 15:50, Kevin Wolf wrote: > Am 10.11.2025 um 16:48 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. >> >> Using s->lock, which is a coroutine mutex, however means that we need to >> take it in a coroutine, so the timer must run in a coroutine. We can >> transform it from the current timer CB style into a coroutine that >> sleeps for the set interval. As a result, however, we can no longer >> just deschedule the timer to instantly guarantee it won’t run anymore, >> but have to await the coroutine’s exit. >> >> (Note even before this patch there were places that may not have been so >> guaranteed after all: Anything calling cache_clean_timer_del() from the >> QEMU main AioContext could have been running concurrently to an existing >> timer CB invocation.) >> >> 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. >> >> (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 >> -static void cache_clean_timer_cb(void *opaque) >> +static void coroutine_fn cache_clean_timer(void *opaque) >> { >> - BlockDriverState *bs = opaque; >> - BDRVQcow2State *s = bs->opaque; >> - qcow2_cache_clean_unused(s->l2_table_cache); >> - qcow2_cache_clean_unused(s->refcount_block_cache); >> - timer_mod(s->cache_clean_timer, qemu_clock_get_ms(QEMU_CLOCK_VIRTUAL) + >> - (int64_t) s->cache_clean_interval * 1000); >> + BDRVQcow2State *s = opaque; >> + uint64_t wait_ns; >> + >> + WITH_QEMU_LOCK_GUARD(&s->lock) { >> + wait_ns = s->cache_clean_interval * NANOSECONDS_PER_SECOND; >> + } >> + >> + while (wait_ns > 0) { >> + qemu_co_sleep_ns_wakeable(&s->cache_clean_timer_wake, >> + QEMU_CLOCK_VIRTUAL, wait_ns); >> + >> + WITH_QEMU_LOCK_GUARD(&s->lock) { >> + if (s->cache_clean_interval > 0) { >> + qcow2_cache_clean_unused(s->l2_table_cache); >> + qcow2_cache_clean_unused(s->refcount_block_cache); >> + } >> + >> + wait_ns = s->cache_clean_interval * NANOSECONDS_PER_SECOND; >> + } >> + } >> + >> + WITH_QEMU_LOCK_GUARD(&s->lock) { >> + s->cache_clean_timer_co = NULL; >> + qemu_co_queue_restart_all(&s->cache_clean_timer_exit); >> + } >> } >> +/** >> + * Delete the cache clean timer and await any yet running instance. >> + * Called holding s->lock. >> + */ >> +static void coroutine_fn >> +cache_clean_timer_co_locked_del_and_wait(BlockDriverState *bs) >> +{ >> + BDRVQcow2State *s = bs->opaque; >> + >> + if (s->cache_clean_timer_co) { >> + s->cache_clean_interval = 0; >> + qemu_co_sleep_wake(&s->cache_clean_timer_wake); >> + qemu_co_queue_wait(&s->cache_clean_timer_exit, &s->lock); >> } >> } > I don't want to count the number of context switches this dance between > cache_clean_timer_co_locked_del_and_wait() and cache_clean_timer() > takes! Good that it's not a hot path. :-) > >> +/* 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; >> >> + /* >> + * We need to stop the cache-clean-timer before destroying the metadata >> + * table caches >> + */ >> + if (s_locked) { >> + cache_clean_timer_co_locked_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 +1312,10 @@ 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 +1327,6 @@ 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)); >> - } >> - >> qapi_free_QCryptoBlockOpenOptions(s->crypto_opts); >> s->crypto_opts = r->crypto_opts; >> } > Is there any specific reason why you move cache_clean_timer_init() > earlier? I don't see an actual problem with the code as it is after this > change, but s->l2_slice_size is related to the cache in a way, so it > would feel safer if the cache cleaner were only started once all the > settings are updated and not only those that it actually happens to > access at the moment. Oh.  I don’t think there’s a good reason.  I think it makes sense to keep the set-up in the old place.  Can you do that in your tree? (I think it’s just because when I split it to stop the timer before deleting the caches, I decided it would conversely make sense to start it after they have been set up.) Hanna