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Thu, 2 Apr 2020 16:47:50 +0000 (UTC) Received: from linux.fritz.box (ovpn-114-227.ams2.redhat.com [10.36.114.227]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 71BC248; Thu, 2 Apr 2020 16:47:46 +0000 (UTC) Date: Thu, 2 Apr 2020 18:47:44 +0200 From: Kevin Wolf To: Dietmar Maurer Subject: Re: bdrv_drained_begin deadlock with io-threads Message-ID: <20200402164744.GE4006@linux.fritz.box> References: <518198448.62.1585671498399@webmail.proxmox.com> <20200401103748.GA4680@linux.fritz.box> <997901084.0.1585755465486@webmail.proxmox.com> <20200401181256.GB27663@linux.fritz.box> <1403939459.52.1585765681569@webmail.proxmox.com> <20200401184431.GD27663@linux.fritz.box> <903538836.29.1585818619688@webmail.proxmox.com> <20200402121403.GB4006@linux.fritz.box> <20200402142524.GD4006@linux.fritz.box> <1043934808.59.1585842050330@webmail.proxmox.com> MIME-Version: 1.0 In-Reply-To: <1043934808.59.1585842050330@webmail.proxmox.com> User-Agent: Mutt/1.12.1 (2019-06-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: quoted-printable Content-Disposition: inline X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.120 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: , Cc: Vladimir Sementsov-Ogievskiy , Stefan Hajnoczi , "qemu-devel@nongnu.org" , qemu-block@nongnu.org, Max Reitz Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 02.04.2020 um 17:40 hat Dietmar Maurer geschrieben: > > Can you reproduce the problem with my script, but pointing it to your > > Debian image and running stress-ng instead of dd?=20 >=20 > yes >=20 > > If so, how long does > > it take to reproduce for you? >=20 > I sometimes need up to 130 iterations ... >=20 > Worse, I thought several times the bug is gone, but then it triggered aga= in (sigh). >=20 > But most times below 30 iteration (if you run "stress-ng -d 5"). >=20 > Also note that it can happen at different places, but always inside a dra= ined section ... I got a stracktrace of a hanging coroutine: (gdb) qemu coroutine 0x7fd8c00132c0 #0 0x00005630e16e9840 in qemu_coroutine_switch (from_=3Dfrom_@entry=3D0x7f= d8c00132c0, to_=3Dto_@entry=3D0x7fd8cda865b8, action=3Daction@entry=3DCOROU= TINE_YIELD) at util/coroutine-ucontext.c:218 #1 0x00005630e16e8521 in qemu_coroutine_yield () at util/qemu-coroutine.c:= 193 #2 0x00005630e16e8ba5 in qemu_co_queue_wait_impl (queue=3Dqueue@entry=3D0x= 5630e48ab580, lock=3Dlock@entry=3D0x0) at util/qemu-coroutine-lock.c:56 #3 0x00005630e161f1ae in blk_wait_while_drained (blk=3Dblk@entry=3D0x5630e= 48ab260) at /home/kwolf/source/qemu/include/qemu/lockable.h:46 #4 0x00005630e1620600 in blk_wait_while_drained (blk=3D0x5630e48ab260) at = block/block-backend.c:1189 #5 0x00005630e1620600 in blk_co_pwritev_part (blk=3D0x5630e48ab260, offset= =3D2922381312, bytes=3D856064, qiov=3Dqiov@entry=3D0x7fd8c002cd70, qiov_off= set=3Dqiov_offset@entry=3D0, flags=3D0) at block/block-backend.c:1189 #6 0x00005630e16207ce in blk_co_pwritev (flags=3D, qiov=3D0= x7fd8c002cd70, bytes=3D, offset=3D, blk=3D) at block/block-backend.c:1429 #7 0x00005630e16207ce in blk_aio_write_entry (opaque=3D0x7fd8c002cdc0) at = block/block-backend.c:1429 #8 0x00005630e16e98bb in coroutine_trampoline (i0=3D, i1=3D= ) at util/coroutine-ucontext.c:115 So I think this is the bug: Calling blk_wait_while_drained() from anywhere between blk_inc_in_flight() and blk_dec_in_flight() is wrong because it will deadlock the drain operation. blk_aio_read/write_entry() take care of this and drop their reference around blk_wait_while_drained(). But if we hit the race condition that drain hasn't yet started there, but it has when we get to blk_co_preadv() or blk_co_pwritev_part(), then we're in a buggy code path. Kevin