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

Michael S. Tsirkin
2024-09-12T14:34:26Z

On Thu, Sep 12, 2024 at 03:48:32PM +0200, Marco Elver wrote:
> On Thu, 12 Sept 2024 at 13:03, Michael S. Tsirkin <mst@redhat.com> 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@syzkaller.appspotmail.com
> > >
> > > ==================================================================
> > > 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.

No, vq->event is not racy.

The race is between a write and a read of event_triggered.
I think data_race tags a read, it can not tag a write, correct?

> 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