From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 5403FCEACEF for ; Mon, 17 Nov 2025 14:51:30 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1vL0ZE-00029n-Py; Mon, 17 Nov 2025 09:50:56 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1vL0Z8-00028O-VA for qemu-devel@nongnu.org; Mon, 17 Nov 2025 09:50:51 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1vL0Z7-0003Eo-18 for qemu-devel@nongnu.org; Mon, 17 Nov 2025 09:50:50 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1763391047; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=9dBTX90l9ueJvnbn+jMaFRARUaVJ9s9Ws67F4xizjk4=; b=C1igZQ+2kYBYATlskPPd+tu2vdJR7PKMIpPVdZkptrHRlCJuzRfSyyuy7OQtbqsQfT4o9z aKL/VDMqetHPlKb+33TJRnALwD3RqvWEXX5xurrToZNoUN7+KApgzoBk/J/NeqTU7IxK7M H0/3FCJ+BZzP0jfPVXV0nrrZAOsSjWc= Received: from mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-301-sfdDatKZMri7-u-EAT3nnQ-1; Mon, 17 Nov 2025 09:50:43 -0500 X-MC-Unique: sfdDatKZMri7-u-EAT3nnQ-1 X-Mimecast-MFC-AGG-ID: sfdDatKZMri7-u-EAT3nnQ_1763391041 Received: from mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.111]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 28125180122F; Mon, 17 Nov 2025 14:50:41 +0000 (UTC) Received: from redhat.com (unknown [10.44.33.115]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 48127180049F; Mon, 17 Nov 2025 14:50:36 +0000 (UTC) Date: Mon, 17 Nov 2025 15:50:33 +0100 From: Kevin Wolf To: Hanna Czenczek Cc: qemu-block@nongnu.org, qemu-devel@nongnu.org, Stefan Hajnoczi , Paolo Bonzini , "Richard W . M . Jones" , Ilya Dryomov , Peter Lieven , Philippe =?iso-8859-1?Q?Mathieu-Daud=E9?= , Alex =?iso-8859-1?Q?Benn=E9e?= , Fam Zheng , Ronnie Sahlberg Subject: Re: [PATCH v2 12/19] qcow2: Fix cache_clean_timer Message-ID: References: <20251110154854.151484-1-hreitz@redhat.com> <20251110154854.151484-13-hreitz@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20251110154854.151484-13-hreitz@redhat.com> X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 Received-SPF: pass client-ip=170.10.129.124; envelope-from=kwolf@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=ham 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 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. Kevin