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Mon, 6 Apr 2020 08:31:49 +0000 (UTC) Received: from linux.fritz.box (ovpn-113-74.ams2.redhat.com [10.36.113.74]) by smtp.corp.redhat.com (Postfix) with ESMTPS id AD324100164D; Mon, 6 Apr 2020 08:31:45 +0000 (UTC) Date: Mon, 6 Apr 2020 10:31:43 +0200 From: Kevin Wolf To: Dietmar Maurer Subject: Re: bdrv_drained_begin deadlock with io-threads Message-ID: <20200406083143.GA6263@linux.fritz.box> References: <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> <20200402171007.GF4006@linux.fritz.box> <2091214547.43.1585902379938@webmail.proxmox.com> <20200403084729.GB5336@linux.fritz.box> <1466731307.175.1585931495885@webmail.proxmox.com> MIME-Version: 1.0 In-Reply-To: <1466731307.175.1585931495885@webmail.proxmox.com> User-Agent: Mutt/1.12.1 (2019-06-15) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 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] X-Received-From: 205.139.110.120 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 03.04.2020 um 18:31 hat Dietmar Maurer geschrieben: >=20 > > On April 3, 2020 10:47 AM Kevin Wolf wrote: > >=20 > > =20 > > Am 03.04.2020 um 10:26 hat Dietmar Maurer geschrieben: > > > > 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.) > > >=20 > > > Wow, that was fast! Seems your patch fixes the bug! > > >=20 > > > I wonder what commit introduced that problem, maybe: > > >=20 > > > https://github.com/qemu/qemu/commit/cf3129323f900ef5ddbccbe86e4fa801e= 88c566e#diff-7cb66df56045598b75a219eebc27efb6 > > >=20 > > > If so, version 4.1.X in not affected by this bug, but 4.2.0 and later= ? > >=20 > > Yes, I'm pretty sure that's the one. >=20 > I also wonder if we can loose an aio_wait_kick() by directly accessing bl= k->in_flight. > I thought this should use atomic_mb_read()? >=20 > diff --git a/block/block-backend.c b/block/block-backend.c > index 8b8f2a80a0..48f3721505 100644 > --- a/block/block-backend.c > +++ b/block/block-backend.c > @@ -2275,7 +2275,7 @@ static bool blk_root_drained_poll(BdrvChild *child) > { > BlockBackend *blk =3D child->opaque; > assert(blk->quiesce_counter); > - return !!blk->in_flight; > + return !!atomic_mb_read(&blk->in_flight); > } I'm not sure if it actually makes a difference (and especially what the barrier is supposed to achieve here), but it would be more consistent with other users of the variable, so I think I agree. Kevin