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([114.247.65.140]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2d022a465f7sm23003015ad.24.2026.07.30.01.40.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 30 Jul 2026 01:40:05 -0700 (PDT) From: Gui-Dong Han To: roger@xenproject.org, xen-devel@lists.xenproject.org Cc: axboe@kernel.dk, linux-block@vger.kernel.org, konrad.wilk@oracle.com, linux-kernel@vger.kernel.org, baijiaju1990@gmail.com, Gui-Dong Han Subject: [PATCH] xen/blkback: Prevent missed completion when draining I/O Date: Thu, 30 Jul 2026 16:40:00 +0800 Message-Id: <20260730084000.3305702-1-hanguidong02@gmail.com> X-Mailer: git-send-email 2.34.1 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" xen_blk_drain_io() sets drain before checking inflight. xen_blkbk_unmap_and_respond_callback() decrements inflight with atomic_dec_and_test() before checking drain. The pre-wait condition check was added to avoid missing a completion, but atomic_set() is unordered. With one I/O in flight, the drain path can set drain to 1 and read inflight as 1 before the completion decrement. The completion path then decrements inflight to 0 but can still read drain as 0. It skips complete(), so the drain path waits until the timeout despite no I/O remaining. Add a full barrier between setting drain and reading inflight. atomic_dec_and_test() already provides full ordering on the completion side. Fixes: 6927d92091df ("xen/blkback: Fix two races in the handling of barrier= requests.") Signed-off-by: Gui-Dong Han --- Found by auditing atomic operations used for synchronization. A similar fix can be found in 6df8e84aa6b5. --- drivers/block/xen-blkback/blkback.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkbac= k/blkback.c index 7871099bc0d4..88526e349ce1 100644 --- a/drivers/block/xen-blkback/blkback.c +++ b/drivers/block/xen-blkback/blkback.c @@ -1021,6 +1021,13 @@ static void xen_blk_drain_io(struct xen_blkif_ring *= ring) struct xen_blkif *blkif =3D ring->blkif; =20 atomic_set(&blkif->drain, 1); + /* + * Publish drain before checking inflight. Otherwise, + * xen_blkbk_unmap_and_respond_callback() can decrement inflight with + * atomic_dec_and_test() and still see drain =3D=3D 0 after this path saw + * inflight > 0, missing the completion. + */ + smp_mb(); do { if (atomic_read(&ring->inflight) =3D=3D 0) break; --=20 2.34.1