linux-kernel - [PATCH v2 1/2] virtio: silence KCSAN warning in virtqueue_get_buf_ctx

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Message-ID: <20260127152524.200465-2-johannes.thumshirn@wdc.com>
Date: Tue, 27 Jan 2026 16:25:22 +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 1/2] virtio: silence KCSAN warning in virtqueue_get_buf_ctx_split

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

 ==================================================================
 BUG: KCSAN: data-race in virtqueue_get_buf_ctx_split+0x6e/0x260

 race at unknown origin, with read to 0xffff8881020f1942 of 2 bytes by task 1 on cpu 7:
  virtqueue_get_buf_ctx_split+0x6e/0x260
  virtqueue_get_buf+0x4b/0x60
  __send_to_port+0x156/0x170
  put_chars+0xcb/0x110
  hvc_console_print+0x1d6/0x2a0
  console_flush_one_record+0x3dd/0x510
  console_unlock+0x8c/0x160
  vprintk_emit+0x2fe/0x380
  vprintk_default+0x1d/0x30
  vprintk+0xe/0x20
  _printk+0x4c/0x60
  btrfs_test_raid_stripe_tree+0x25/0x90
  btrfs_run_sanity_tests.cold+0xf1/0x13b
  init_btrfs_fs+0x73/0x110
  do_one_initcall+0x5b/0x2d0
  kernel_init_freeable+0x2a2/0x340
  kernel_init+0x1e/0x1b0
  ret_from_fork+0x137/0x1b0
  ret_from_fork_asm+0x1a/0x30

 value changed: 0x0160 -> 0x0161

 Reported by Kernel Concurrency Sanitizer on:
 CPU: 7 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.19.0-rc7+ #219 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 more_used_split() with data_race() to silence
the warning.

Signed-off-by: Johannes Thumshirn <johannes.thumshirn@....com>
---
 drivers/virtio/virtio_ring.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index ddab68959671..1db27ee2d89f 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -808,8 +808,8 @@ static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
 
 static bool more_used_split(const struct vring_virtqueue *vq)
 {
-	return vq->last_used_idx != virtio16_to_cpu(vq->vq.vdev,
-			vq->split.vring.used->idx);
+	return data_race(vq->last_used_idx != virtio16_to_cpu(vq->vq.vdev,
+				vq->split.vring.used->idx));
 }
 
 static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
-- 
2.52.0