linux-kernel - [PATCH v2 2/2] virtio: silence KCSAN warning in virtqueue_kick

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Message-ID: <20260127152524.200465-3-johannes.thumshirn@wdc.com>
Date: Tue, 27 Jan 2026 16:25:23 +0100
From: Johannes Thumshirn <johannes.thumshirn@....com>
To: virtualization@...ts.linux.dev
Cc: "Michael S . Tsirkin" <mst@...hat.com>,
	Jason Wang <jasowang@...hat.com>,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
	Eugenio Pérez <eperezma@...hat.com>,
	Alexander Graf <graf@...zon.com>,
	linux-kernel@...r.kernel.org,
	Johannes Thumshirn <johannes.thumshirn@....com>
Subject: [PATCH v2 2/2] 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 0xffff888101c9cd44 of 2 bytes by task 224 on cpu 2:
  virtqueue_kick_prepare+0x14d/0x200
  virtio_fs_enqueue_req+0x664/0x7d0
  virtio_fs_send_req+0xac/0x230
  flush_bg_queue+0x1a8/0x1f0
  fuse_simple_background+0x312/0x490
  fuse_file_put+0xcf/0x190
  fuse_file_release+0xd3/0xf0
  fuse_release+0x91/0xb0
  __fput+0x200/0x4f0
  ____fput+0x15/0x20
  task_work_run+0xda/0x140
  do_exit+0x414/0x11a0
  do_group_exit+0x53/0xf0
  __x64_sys_exit_group+0x25/0x30
  x64_sys_call+0x1c23/0x1c30
  do_syscall_64+0x5d/0x240
  entry_SYSCALL_64_after_hwframe+0x76/0x7e

 value changed: 0x1ab0 -> 0x1ab3

 Reported by Kernel Concurrency Sanitizer on:
 CPU: 2 UID: 0 PID: 224 Comm: grepconf.sh Not tainted 6.19.0-rc7+ #230 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>
---
 include/uapi/linux/virtio_ring.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/uapi/linux/virtio_ring.h b/include/uapi/linux/virtio_ring.h
index f8c20d3de8da..32568cfa1c63 100644
--- a/include/uapi/linux/virtio_ring.h
+++ b/include/uapi/linux/virtio_ring.h
@@ -194,7 +194,7 @@ struct vring {
 /* We publish the used event index at the end of the available ring, and vice
  * versa. They are at the end for backwards compatibility. */
 #define vring_used_event(vr) ((vr)->avail->ring[(vr)->num])
-#define vring_avail_event(vr) (*(__virtio16 *)&(vr)->used->ring[(vr)->num])
+#define vring_avail_event(vr) (data_race(*(__virtio16 *)&(vr)->used->ring[(vr)->num]))
 
 static inline void vring_init(struct vring *vr, unsigned int num, void *p,
 			      unsigned long align)
-- 
2.52.0