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 <[email protected]> --- 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
