linux-kernel - [PATCH RFC 1/3] virtio: silence KCSAN warning in virtqueue_kick

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Message-ID: <20260127083926.865555-2-johannes.thumshirn@wdc.com>
Date: Tue, 27 Jan 2026 09:39:24 +0100
From: Johannes Thumshirn <johannes.thumshirn@....com>
To: virtualization@...ts.linux.dev
Cc: Alexander Graf <graf@...zon.com>,
	mst@...hat.com,
	jasowang@...hat.com,
	xuanzhuo@...ux.alibaba.com,
	eperezma@...hat.com,
	linux-kernel@...r.kernel.org,
	Johannes Thumshirn <johannes.thumshirn@....com>
Subject: [PATCH RFC 1/3] virtio: silence KCSAN warning in virtqueue_kick_prepare

When booting a Qemu VM with virtio-blk and KCSAN enabled, KCSAN emits
the following warning about a data-race in virtqueue_kick_prepare().

 ==================================================================
 BUG: KCSAN: data-race in virtqueue_kick_prepare+0x14d/0x200

 race at unknown origin, with read to 0xffff888101cd6d44 of 2 bytes by task 162 on cpu 0:
  virtqueue_kick_prepare+0x14d/0x200
  virtio_fs_enqueue_req+0x664/0x7d0
  virtio_fs_send_req+0xac/0x230
  __fuse_simple_request+0x1e1/0x580
  fuse_readlink_folio+0x1dc/0x2a0
  fuse_get_link+0xd6/0x190
  vfs_get_link+0x70/0x80
  ovl_get_link+0x4d/0x90
  pick_link+0x2c6/0x6a0
  step_into_slowpath+0x2ea/0x590
  path_openat+0x5bf/0x1b30
  do_filp_open+0x177/0x260
  do_sys_openat2+0x69/0xc0
  __x64_sys_openat+0x7a/0xc0
  x64_sys_call+0x10cf/0x1c30
  do_syscall_64+0x5d/0x240
  entry_SYSCALL_64_after_hwframe+0x76/0x7e

 value changed: 0x0e63 -> 0x0e65

 Reported by Kernel Concurrency Sanitizer on:
 CPU: 0 UID: 0 PID: 162 Comm: systemctl Not tainted 6.19.0-rc7+ #220 PREEMPT(none)
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-9.fc43 06/10/2025
 ==================================================================

This warning is likely a false positive as the change happens on the
virtio vring.

Annotate the return of vring_avail_event() with data_race() to silence
the warning.

Signed-off-by: Johannes Thumshirn <johannes.thumshirn@....com>
---
 drivers/virtio/virtio_ring.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index ddab68959671..5f8c8254bb6c 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -736,9 +736,10 @@ static bool virtqueue_kick_prepare_split(struct virtqueue *_vq)
 	LAST_ADD_TIME_INVALID(vq);
 
 	if (vq->event) {
-		needs_kick = vring_need_event(virtio16_to_cpu(_vq->vdev,
-					vring_avail_event(&vq->split.vring)),
-					      new, old);
+		needs_kick = vring_need_event(
+			virtio16_to_cpu(_vq->vdev, data_race(vring_avail_event(
+							   &vq->split.vring))),
+			new, old);
 	} else {
 		needs_kick = !(vq->split.vring.used->flags &
 					cpu_to_virtio16(_vq->vdev,
-- 
2.52.0