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dkim=none (message not signed) header.d=none;redhat.com; dmarc=none action=none header.from=virtuozzo.com; Received: from VI1PR08MB5503.eurprd08.prod.outlook.com (2603:10a6:803:137::19) by VI1PR08MB4269.eurprd08.prod.outlook.com (2603:10a6:803:f5::11) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3632.21; Tue, 8 Dec 2020 15:33:27 +0000 Received: from VI1PR08MB5503.eurprd08.prod.outlook.com ([fe80::44c9:5ac7:5302:7179]) by VI1PR08MB5503.eurprd08.prod.outlook.com ([fe80::44c9:5ac7:5302:7179%4]) with mapi id 15.20.3632.023; Tue, 8 Dec 2020 15:33:27 +0000 Subject: Re: [PATCH 3/3] block: Fix deadlock in bdrv_co_yield_to_drain() To: Kevin Wolf , qemu-block@nongnu.org Cc: qemu-devel@nongnu.org, qemu-stable@nongnu.org, stefanha@redhat.com, mreitz@redhat.com References: <20201203172311.68232-1-kwolf@redhat.com> <20201203172311.68232-4-kwolf@redhat.com> From: Vladimir Sementsov-Ogievskiy Message-ID: Date: Tue, 8 Dec 2020 18:33:25 +0300 User-Agent: Mozilla/5.0 (X11; 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envelope-from=vsementsov@virtuozzo.com; helo=EUR04-HE1-obe.outbound.protection.outlook.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, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, MSGID_FROM_MTA_HEADER=0.001, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-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: 2.1.23 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" 03.12.2020 20:23, Kevin Wolf wrote: > If bdrv_co_yield_to_drain() is called for draining a block node that > runs in a different AioContext, it keeps that AioContext locked while it > yields and schedules a BH in the AioContext to do the actual drain. > > As long as executing the BH is the very next thing the event loop of the s/thing the event/thing in the event/ (I've reread several times to understand :) > node's AioContext, this actually happens to work, but when it tries to > execute something else that wants to take the AioContext lock, it will > deadlock. (In the bug report, this other thing is a virtio-scsi device > running virtio_scsi_data_plane_handle_cmd().) > > Instead, always drop the AioContext lock across the yield and reacquire > it only when the coroutine is reentered. The BH needs to unconditionally > take the lock for itself now. > > This fixes the 'block_resize' QMP command on a block node that runs in > an iothread. > > Cc: qemu-stable@nongnu.org > Fixes: eb94b81a94bce112e6b206df846c1551aaf6cab6 > Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1903511 > Signed-off-by: Kevin Wolf I don't feel myself good enough in aio contexts acquiring and switching, to see any side effects. At least I don't see any obvious mistakes, so my weak: Reviewed-by: Vladimir Sementsov-Ogievskiy Note, I looked through the callers: bdrv_do_drained_begin/end should be ok, as their normal usage is to start/end drained section under acquired aio context, so it seems correct to temporary release the context. Still I didn't check all drained sections in the code. bdrv_drain_all_begin seems OK too (we just wait until everything is drained, not bad to temporary release the lock). Still I don't see any call of it from coroutine context. -- Best regards, Vladimir