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From: Sasha Levin <sashal@kernel.org>
To: linux-kernel@vger.kernel.org, stable@vger.kernel.org
Cc: Parav Pandit <parav@nvidia.com>,
	"Michael S . Tsirkin" <mst@redhat.com>,
	Sasha Levin <sashal@kernel.org>,
	virtualization@lists.linux-foundation.org
Subject: [PATCH AUTOSEL 5.4 02/10] virtio: Improve vq->broken access to avoid any compiler optimization
Date: Mon, 23 Aug 2021 20:54:49 -0400	[thread overview]
Message-ID: <20210824005458.631377-2-sashal@kernel.org> (raw)
In-Reply-To: <20210824005458.631377-1-sashal@kernel.org>

From: Parav Pandit <parav@nvidia.com>

[ Upstream commit 60f0779862e4ab943810187752c462e85f5fa371 ]

Currently vq->broken field is read by virtqueue_is_broken() in busy
loop in one context by virtnet_send_command().

vq->broken is set to true in other process context by
virtio_break_device(). Reader and writer are accessing it without any
synchronization. This may lead to a compiler optimization which may
result to optimize reading vq->broken only once.

Hence, force reading vq->broken on each invocation of
virtqueue_is_broken() and also force writing it so that such
update is visible to the readers.

It is a theoretical fix that isn't yet encountered in the field.

Signed-off-by: Parav Pandit <parav@nvidia.com>
Link: https://lore.kernel.org/r/20210721142648.1525924-2-parav@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/virtio/virtio_ring.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 97e8a195e18f..c9550b7291e7 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -2262,7 +2262,7 @@ bool virtqueue_is_broken(struct virtqueue *_vq)
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
 
-	return vq->broken;
+	return READ_ONCE(vq->broken);
 }
 EXPORT_SYMBOL_GPL(virtqueue_is_broken);
 
@@ -2276,7 +2276,9 @@ void virtio_break_device(struct virtio_device *dev)
 
 	list_for_each_entry(_vq, &dev->vqs, list) {
 		struct vring_virtqueue *vq = to_vvq(_vq);
-		vq->broken = true;
+
+		/* Pairs with READ_ONCE() in virtqueue_is_broken(). */
+		WRITE_ONCE(vq->broken, true);
 	}
 }
 EXPORT_SYMBOL_GPL(virtio_break_device);
-- 
2.30.2


  reply	other threads:[~2021-08-24  0:59 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-24  0:54 [PATCH AUTOSEL 5.4 01/10] opp: remove WARN when no valid OPPs remain Sasha Levin
2021-08-24  0:54 ` Sasha Levin [this message]
2021-08-24  0:54 ` [PATCH AUTOSEL 5.4 03/10] virtio_pci: Support surprise removal of virtio pci device Sasha Levin
2021-08-24  0:54 ` [PATCH AUTOSEL 5.4 04/10] vringh: Use wiov->used to check for read/write desc order Sasha Levin
2021-08-24  0:54 ` [PATCH AUTOSEL 5.4 05/10] qed: qed ll2 race condition fixes Sasha Levin
2021-08-24  0:54 ` [PATCH AUTOSEL 5.4 06/10] qed: Fix null-pointer dereference in qed_rdma_create_qp() Sasha Levin
2021-08-24  0:54 ` [PATCH AUTOSEL 5.4 07/10] drm: Copy drm_wait_vblank to user before returning Sasha Levin
2021-08-24  0:54 ` [PATCH AUTOSEL 5.4 08/10] drm/nouveau/disp: power down unused DP links during init Sasha Levin
2021-08-24  0:54 ` [PATCH AUTOSEL 5.4 09/10] drm/nouveau: block a bunch of classes from userspace Sasha Levin
2021-08-24  0:54 ` [PATCH AUTOSEL 5.4 10/10] net/rds: dma_map_sg is entitled to merge entries Sasha Levin

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