Re: [PATCH v2] virtio_ring: Fix data races in virtqueue_enable_cb_delayed_split()

[Chaohai Chen <wdhh6@aliyun.com>]

On Fri, Mar 06, 2026 at 05:04:11AM -0500, Michael S. Tsirkin wrote:
> here's how I would put it:
> 
> KCSAN reports .....
> this is because ....
> 
> 
> This is not a real issue, because ....
> To disable the KCSAN warning, use READ_ONCE to read vq->split.vring.used->idx.
> 
> 
> 
> 
> On Fri, Mar 06, 2026 at 10:24:48AM +0800, Chaohai Chen wrote:
> > Add READ_ONCE when fetching vq->split.vring.used->idx
> > without synchronization.
> 
> it's with synchronization, of course
> 
> > KCSAN detected
> 
> detected -> incorrectly reported
> 
> > data races when accessing the split virtqueue's
> > used ring, which is shared memory concurrently accessed by both the CPU
> > and the virtio device (hypervisor).
> > 
> > Example KCSAN report:
> > 
> > [  109.277250] ==================================================================
> > [  109.283600] BUG: KCSAN: data-race in virtqueue_enable_cb_delayed_split+0x10f/0x170
> > 
> > [  109.295263] race at unknown origin, with read to 0xffff8b2a92ef2042 of 2 bytes by interrupt on cpu 1:
> > [  109.306934]  virtqueue_enable_cb_delayed_split+0x10f/0x170
> > [  109.312880]  virtqueue_enable_cb_delayed+0x3b/0x70
> > [  109.318852]  start_xmit+0x315/0x860 [virtio_net]
> > [  109.324532]  dev_hard_start_xmit+0x85/0x380
> > [  109.329993]  sch_direct_xmit+0xd3/0x680
> > [  109.335360]  __dev_xmit_skb+0x4ee/0xcc0
> > [  109.340568]  __dev_queue_xmit+0x560/0xe00
> > [  109.345701]  ip_finish_output2+0x49a/0x9b0
> > [  109.350743]  __ip_finish_output+0x131/0x250
> > [  109.355789]  ip_finish_output+0x28/0x180
> > [  109.360712]  ip_output+0xa0/0x1c0
> > [  109.365479]  __ip_queue_xmit+0x68d/0x9e0
> > [  109.370156]  ip_queue_xmit+0x33/0x40
> > [  109.374783]  __tcp_transmit_skb+0x1703/0x1970
> > [  109.379467]  __tcp_send_ack.part.0+0x1bb/0x320
> > ...
> > [  109.499585]  do_idle+0x7a/0xe0
> > [  109.502979]  cpu_startup_entry+0x25/0x30
> > [  109.506481]  start_secondary+0x116/0x150
> > [  109.509930]  common_startup_64+0x13e/0x141
> > 
> > [  109.516626] value changed: 0x0029 -> 0x002a
> > ================================================================================
> > 
> > Signed-off-by: Chaohai Chen <wdhh6@aliyun.com>
> 
> 
> 
> I'd like to see code size before/after compared, and included in
> the log. Does this make the compiler's work much harder?
> 
I found the code size is the same. And virtio_store_mb() can do sync.
Perhaps we don't need to modify the code here, but rather fix KCSAN.