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Thu, 2 Apr 2020 17:10:15 +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 1C9A260BF1; Thu, 2 Apr 2020 17:10:08 +0000 (UTC) Date: Thu, 2 Apr 2020 19:10:07 +0200 From: Kevin Wolf To: Dietmar Maurer Subject: Re: bdrv_drained_begin deadlock with io-threads Message-ID: <20200402171007.GF4006@linux.fritz.box> References: <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> <20200402164744.GE4006@linux.fritz.box> MIME-Version: 1.0 In-Reply-To: <20200402164744.GE4006@linux.fritz.box> User-Agent: Mutt/1.12.1 (2019-06-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 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: qemu-block@nongnu.org, Vladimir Sementsov-Ogievskiy , "qemu-devel@nongnu.org" , Stefan Hajnoczi , Max Reitz Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 02.04.2020 um 18:47 hat Kevin Wolf geschrieben: > 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 a= gain (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 d= rained section ... >=20 > I got a stracktrace of a hanging coroutine: >=20 > (gdb) qemu coroutine 0x7fd8c00132c0 > #0 0x00005630e16e9840 in qemu_coroutine_switch (from_=3Dfrom_@entry=3D0x= 7fd8c00132c0, to_=3Dto_@entry=3D0x7fd8cda865b8, action=3Daction@entry=3DCOR= OUTINE_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=3D= 0x5630e48ab580, lock=3Dlock@entry=3D0x0) at util/qemu-coroutine-lock.c:56 > #3 0x00005630e161f1ae in blk_wait_while_drained (blk=3Dblk@entry=3D0x563= 0e48ab260) at /home/kwolf/source/qemu/include/qemu/lockable.h:46 > #4 0x00005630e1620600 in blk_wait_while_drained (blk=3D0x5630e48ab260) a= t block/block-backend.c:1189 > #5 0x00005630e1620600 in blk_co_pwritev_part (blk=3D0x5630e48ab260, offs= et=3D2922381312, bytes=3D856064, qiov=3Dqiov@entry=3D0x7fd8c002cd70, qiov_o= ffset=3Dqiov_offset@entry=3D0, flags=3D0) > at block/block-backend.c:1189 > #6 0x00005630e16207ce in blk_co_pwritev (flags=3D, qiov= =3D0x7fd8c002cd70, bytes=3D, offset=3D, blk= =3D) at block/block-backend.c:1429 > #7 0x00005630e16207ce in blk_aio_write_entry (opaque=3D0x7fd8c002cdc0) a= t block/block-backend.c:1429 > #8 0x00005630e16e98bb in coroutine_trampoline (i0=3D, i1= =3D) at util/coroutine-ucontext.c:115 >=20 > 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. >=20 > 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. With the following patch, it seems to survive for now. I'll give it some more testing tomorrow (also qemu-iotests to check that I didn't accidentally break something else.) Kevin diff --git a/block/block-backend.c b/block/block-backend.c index 8b8f2a80a0..634acd1541 100644 --- a/block/block-backend.c +++ b/block/block-backend.c @@ -1143,7 +1143,9 @@ static int blk_check_byte_request(BlockBackend *blk, = int64_t offset, static void coroutine_fn blk_wait_while_drained(BlockBackend *blk) { if (blk->quiesce_counter && !blk->disable_request_queuing) { + blk_dec_in_flight(blk); qemu_co_queue_wait(&blk->queued_requests, NULL); + blk_inc_in_flight(blk); } } =20 @@ -1154,8 +1156,6 @@ int coroutine_fn blk_co_preadv(BlockBackend *blk, int= 64_t offset, int ret; BlockDriverState *bs; =20 - blk_wait_while_drained(blk); - /* Call blk_bs() only after waiting, the graph may have changed */ bs =3D blk_bs(blk); trace_blk_co_preadv(blk, bs, offset, bytes, flags); @@ -1186,8 +1186,6 @@ int coroutine_fn blk_co_pwritev_part(BlockBackend *bl= k, int64_t offset, int ret; BlockDriverState *bs; =20 - blk_wait_while_drained(blk); - /* Call blk_bs() only after waiting, the graph may have changed */ bs =3D blk_bs(blk); trace_blk_co_pwritev(blk, bs, offset, bytes, flags); @@ -1262,6 +1260,7 @@ static int blk_prw(BlockBackend *blk, int64_t offset,= uint8_t *buf, .ret =3D NOT_DONE, }; =20 + blk_inc_in_flight(blk); if (qemu_in_coroutine()) { /* Fast-path if already in coroutine context */ co_entry(&rwco); @@ -1270,6 +1269,7 @@ static int blk_prw(BlockBackend *blk, int64_t offset,= uint8_t *buf, bdrv_coroutine_enter(blk_bs(blk), co); BDRV_POLL_WHILE(blk_bs(blk), rwco.ret =3D=3D NOT_DONE); } + blk_dec_in_flight(blk); =20 return rwco.ret; } @@ -1388,9 +1388,7 @@ static void blk_aio_read_entry(void *opaque) QEMUIOVector *qiov =3D rwco->iobuf; =20 if (rwco->blk->quiesce_counter) { - blk_dec_in_flight(rwco->blk); blk_wait_while_drained(rwco->blk); - blk_inc_in_flight(rwco->blk); } =20 assert(qiov->size =3D=3D acb->bytes); @@ -1406,9 +1404,7 @@ static void blk_aio_write_entry(void *opaque) QEMUIOVector *qiov =3D rwco->iobuf; =20 if (rwco->blk->quiesce_counter) { - blk_dec_in_flight(rwco->blk); blk_wait_while_drained(rwco->blk); - blk_inc_in_flight(rwco->blk); } =20 assert(!qiov || qiov->size =3D=3D acb->bytes);