linux-kernel - Re: [PATCH] virtio_ring: tag event

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Message-ID: <CANpmjNMNgvg+NKtzxy06AB4QFQKzFsmp-chB1G_+XueiKNA-bA@mail.gmail.com>
Date: Thu, 12 Sep 2024 16:50:02 +0200
From: Marco Elver <elver@...gle.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: linux-kernel@...r.kernel.org, 
	syzbot+8a02104389c2e0ef5049@...kaller.appspotmail.com, 
	Jason Wang <jasowang@...hat.com>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, 
	Eugenio Pérez <eperezma@...hat.com>, 
	virtualization@...ts.linux.dev
Subject: Re: [PATCH] virtio_ring: tag event_triggered as racy for KCSAN

On Thu, 12 Sept 2024 at 16:45, Michael S. Tsirkin <mst@...hat.com> wrote:
>
> event_triggered is fundamentally racy. There are races of 2 types:
> 1. vq processing can read false value while interrupt
>    triggered and set it to true.
>    result will be a bit of extra work when disabling cbs, no big deal.
>
> 1. vq processing can set false value then interrupt
>    immediately sets true value
>    since interrupt then triggers a callback which will
>    process buffers, this is also not an issue.
>
> However, looks like KCSAN isn't smart enough to figure this out.
> Tag the field __data_racy for now.
> We should probably look at ways to make this more straight-forwardly
> correct.
>
> Cc: Marco Elver <elver@...gle.com>
> Reported-by: syzbot+8a02104389c2e0ef5049@...kaller.appspotmail.com
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> ---
>  drivers/virtio/virtio_ring.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index be7309b1e860..724aa9c27c6b 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -194,7 +194,7 @@ struct vring_virtqueue {
>         u16 last_used_idx;
>
>         /* Hint for event idx: already triggered no need to disable. */
> -       bool event_triggered;
> +       bool __data_racy event_triggered;

I guess if you don't care about any data races on this variable, this
is reasonable. Although note that data race is more subtle than just a
"race": https://lwn.net/Articles/816850/

Acked-by: Marco Elver <elver@...gle.com>