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Tue, 7 Apr 2020 08:59:21 +0000 (UTC) Received: from linux.fritz.box (ovpn-113-253.ams2.redhat.com [10.36.113.253]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 73903277A5; Tue, 7 Apr 2020 08:59:16 +0000 (UTC) Date: Tue, 7 Apr 2020 10:59:15 +0200 From: Kevin Wolf To: Vladimir Sementsov-Ogievskiy Subject: Re: [PATCH for-5.0 v2 3/3] block: Fix blk->in_flight during blk_wait_while_drained() Message-ID: <20200407085915.GB7695@linux.fritz.box> References: <20200406171403.6761-1-kwolf@redhat.com> <20200406171403.6761-4-kwolf@redhat.com> <3c8fd1b8-5138-b6eb-1e9d-55e03ad078c7@virtuozzo.com> MIME-Version: 1.0 In-Reply-To: <3c8fd1b8-5138-b6eb-1e9d-55e03ad078c7@virtuozzo.com> User-Agent: Mutt/1.12.1 (2019-06-15) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 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, s.reiter@proxmox.com, qemu-devel@nongnu.org, dietmar@proxmox.com, stefanha@redhat.com, mreitz@redhat.com, t.lamprecht@proxmox.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 07.04.2020 um 08:52 hat Vladimir Sementsov-Ogievskiy geschrieben: > 06.04.2020 20:14, Kevin Wolf wrote: > > Waiting in blk_wait_while_drained() while blk->in_flight is increased > > for the current request is wrong because it will cause the drain > > operation to deadlock. > >=20 > > This patch makes sure that blk_wait_while_drained() is called with > > blk->in_flight increased exactly once for the current request, and that > > it temporarily decreases the counter while it waits. > >=20 > > Fixes: cf3129323f900ef5ddbccbe86e4fa801e88c566e > > Signed-off-by: Kevin Wolf > > --- > > block/block-backend.c | 17 +++++------------ > > 1 file changed, 5 insertions(+), 12 deletions(-) > >=20 > > diff --git a/block/block-backend.c b/block/block-backend.c > > index d330e08b05..f621435f0b 100644 > > --- a/block/block-backend.c > > +++ b/block/block-backend.c > > @@ -1140,10 +1140,15 @@ static int blk_check_byte_request(BlockBackend = *blk, int64_t offset, > > return 0; > > } > > +/* To be called between exactly one pair of blk_inc/dec_in_flight() */ > > static void coroutine_fn blk_wait_while_drained(BlockBackend *blk) > > { > > + assert(blk->in_flight > 0); >=20 > Hmm. You promise to make sure that in_flight increased exactly once. > Shouldn't it be assert(blk->in_flight =3D=3D 1) ? Exactly once for this specific request, but if you have multiple requests in flight, blk->in_flight will be the sum of all requests. Just asserting > 0 should still catch potential bugs because you won't always have multiple requests in flight. > > + > > 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); > > } > > } > > @@ -1416,12 +1421,6 @@ static void blk_aio_read_entry(void *opaque) > > BlkRwCo *rwco =3D &acb->rwco; > > QEMUIOVector *qiov =3D rwco->iobuf; > > - if (rwco->blk->quiesce_counter) { > > - blk_dec_in_flight(rwco->blk); > > - blk_wait_while_drained(rwco->blk); > > - blk_inc_in_flight(rwco->blk); > > - } >=20 > Hm, you drop it as it's called from blk_do_preadv too. I think it > worth mentioning in commit message still. Okay, I can add a sentence like "The blk_wait_while_drained() call in blk_aio_read/write_entry is redundant with the one in blk_co_*(), so drop it." > > - > > assert(qiov->size =3D=3D acb->bytes); > > rwco->ret =3D blk_do_preadv(rwco->blk, rwco->offset, acb->bytes, > > qiov, rwco->flags); > > @@ -1434,12 +1433,6 @@ static void blk_aio_write_entry(void *opaque) > > BlkRwCo *rwco =3D &acb->rwco; > > QEMUIOVector *qiov =3D rwco->iobuf; > > - if (rwco->blk->quiesce_counter) { > > - blk_dec_in_flight(rwco->blk); > > - blk_wait_while_drained(rwco->blk); > > - blk_inc_in_flight(rwco->blk); > > - } > > - > > assert(!qiov || qiov->size =3D=3D acb->bytes); > > rwco->ret =3D blk_do_pwritev_part(rwco->blk, rwco->offset, acb->b= ytes, > > qiov, 0, rwco->flags); > >=20 >=20 > With assert(blk->in_flight =3D=3D 1) and mention extra wait removing in c= ommit message: > Reviewed-by: Vladimir Sementsov-Ogievskiy Thanks, and I hope you agree with blk->in_flight > 0 now. Kevin