LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
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 4.9 1/3] virtio: Improve vq->broken access to avoid any compiler optimization
Date: Mon, 23 Aug 2021 20:55:37 -0400	[thread overview]
Message-ID: <20210824005539.631820-1-sashal@kernel.org> (raw)

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 5cad9f41c238..cf7eccfe3469 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -1150,7 +1150,7 @@ 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);
 
@@ -1164,7 +1164,9 @@ 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  1:02 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-24  0:55 Sasha Levin [this message]
2021-08-24  0:55 ` [PATCH AUTOSEL 4.9 2/3] vringh: Use wiov->used to check for read/write desc order Sasha Levin
2021-08-24  0:55 ` [PATCH AUTOSEL 4.9 3/3] net/rds: dma_map_sg is entitled to merge entries Sasha Levin

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20210824005539.631820-1-sashal@kernel.org \
    --to=sashal@kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mst@redhat.com \
    --cc=parav@nvidia.com \
    --cc=stable@vger.kernel.org \
    --cc=virtualization@lists.linux-foundation.org \
    --subject='Re: [PATCH AUTOSEL 4.9 1/3] virtio: Improve vq->broken access to avoid any compiler optimization' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).