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Tue, 19 May 2020 11:04:02 +0000 (UTC) Date: Tue, 19 May 2020 13:04:01 +0200 From: Kevin Wolf To: Vladimir Sementsov-Ogievskiy Subject: Re: [PATCH v2 5/9] block/io: expand in_flight inc/dec section: simple cases Message-ID: <20200519110401.GJ7652@linux.fritz.box> References: <20200427143907.5710-1-vsementsov@virtuozzo.com> <20200427143907.5710-6-vsementsov@virtuozzo.com> MIME-Version: 1.0 In-Reply-To: <20200427143907.5710-6-vsementsov@virtuozzo.com> 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-Disposition: inline Received-SPF: pass client-ip=205.139.110.120; envelope-from=kwolf@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/05/18 23:56:10 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] 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, DKIMWL_WL_HIGH=0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_PASS=-0.001, URIBL_BLOCKED=0.001 autolearn=_AUTOLEARN 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: , Cc: fam@euphon.net, qemu-block@nongnu.org, qemu-devel@nongnu.org, mreitz@redhat.com, stefanha@redhat.com, den@openvz.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 27.04.2020 um 16:39 hat Vladimir Sementsov-Ogievskiy geschrieben: > It's safer to expand in_flight request to start before enter to > coroutine in synchronous wrappers, due to the following (theoretical) > problem: > > Consider write. > It's possible, that qemu_coroutine_enter only schedules execution, > assume such case. > > Then we may possibly have the following: > > 1. Somehow check that we are not in drained section in outer code. > > 2. Call bdrv_pwritev(), assuming that it will increase in_flight, which > will protect us from starting drained section. > > 3. It calls bdrv_prwv_co() -> bdrv_coroutine_enter() (not yet increased > in_flight). > > 4. Assume coroutine not yet actually entered, only scheduled, and we go > to some code, which starts drained section (as in_flight is zero). > > 5. Scheduled coroutine starts, and blindly increases in_flight, and we > are in drained section with in_flight request. > > Signed-off-by: Vladimir Sementsov-Ogievskiy > diff --git a/block/io.c b/block/io.c > index 061f3f2590..a91d8c1e21 100644 > --- a/block/io.c > +++ b/block/io.c > @@ -1511,7 +1511,8 @@ int coroutine_fn bdrv_co_preadv(BdrvChild *child, > return bdrv_co_preadv_part(child, offset, bytes, qiov, 0, flags); > } > > -int coroutine_fn bdrv_co_preadv_part(BdrvChild *child, > +/* To be called between exactly one pair of bdrv_inc/dec_in_flight() */ You have lots of comments like this one. Isn't this condition too strict, though? In the BlockBackend layer, it needs to be true because blk_wait_while_drained() decreases in_flight only once (which is an ugly hack, honestly, but it works...). It's comparable to how AIO_WAIT_WHILE() relies on having locked the context exactly once even though it is a recursive lock, because it wants to drop the lock temporarily. I don't think the same reasoning applies to BDS in_flight, does it? We can potentially simplify the code if we don't have to fulfill the condition. Kevin