Re: [syzbot] [virt?] KCSAN: data-race in virtqueue_disable_cb / vring_interrupt (4)

[Marco Elver]

On Thu, 12 Sept 2024 at 13:03, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> On Thu, Sep 12, 2024 at 01:11:21AM -0700, syzbot wrote:
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    7c6a3a65ace7 minmax: reduce min/max macro expansion in ato..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=1608e49f980000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=1e7d02549be622b2
> > dashboard link: https://syzkaller.appspot.com/bug?extid=8a02104389c2e0ef5049
> > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> >
> > Unfortunately, I don't have any reproducer for this issue yet.
> >
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/a1f7496fa21f/disk-7c6a3a65.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/f423739e51a9/vmlinux-7c6a3a65.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/b65a0f38cbd7/bzImage-7c6a3a65.xz
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+8a02104389c2e0ef5049@xxxxxxxxxxxxxxxxxxxxxxxxx
> >
> > ==================================================================
> > BUG: KCSAN: data-race in virtqueue_disable_cb / vring_interrupt
> >
> > write to 0xffff88810285ef52 of 1 bytes by interrupt on cpu 0:
> >  vring_interrupt+0x12b/0x180 drivers/virtio/virtio_ring.c:2591
>
>
> Yes, it's racy!
>
> 2589:        /* Just a hint for performance: so it's ok that this can be racy! */
> 2590:        if (vq->event)
> 2591:            vq->event_triggered = true;
>
>
> Question: is there a way to annotate code to tell syzbot it's ok?

In this case, "if (data_race(vq->event))" might be the right choice.

This is a quick guide on which access primitive to use in concurrent
code: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/access-marking.txt

Thanks,
-- Marco