linux-kernel - Re: [PATCH] virtio_ring: Fix the stale index in available ring

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Date: Wed, 27 Mar 2024 09:14:47 +1000
From: Gavin Shan <gshan@...hat.com>
To: Will Deacon <will@...nel.org>, Keir Fraser <keirf@...gle.com>
Cc: "Michael S. Tsirkin" <mst@...hat.com>, virtualization@...ts.linux.dev,
 linux-kernel@...r.kernel.org, jasowang@...hat.com,
 xuanzhuo@...ux.alibaba.com, yihyu@...hat.com, shan.gavin@...il.com,
 linux-arm-kernel@...ts.infradead.org,
 Catalin Marinas <catalin.marinas@....com>, mochs@...dia.com
Subject: Re: [PATCH] virtio_ring: Fix the stale index in available ring

On 3/27/24 01:46, Will Deacon wrote:
> On Tue, Mar 26, 2024 at 11:43:13AM +0000, Will Deacon wrote:
> 
> Ok, long shot after eyeballing the vhost code, but does the diff below
> help at all? It looks like vhost_vq_avail_empty() can advance the value
> saved in 'vq->avail_idx' but without the read barrier, possibly confusing
> vhost_get_vq_desc() in polling mode.
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 045f666b4f12..87bff710331a 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -2801,6 +2801,7 @@ bool vhost_vq_avail_empty(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>                  return false;
>          vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
>   
> +       smp_rmb();
>          return vq->avail_idx == vq->last_avail_idx;
>   }
>   EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);
> 

Thanks, Will. I already noticed smp_rmb() has been missed in vhost_vq_avail_empty().
The issue still exists after smp_rmb() is added here. However, I'm inspired by your
suggestion and recheck the code again. It seems another smp_rmb() has been missed
in vhost_enable_notify().

With smp_rmb() added to vhost_vq_avail_empty() and vhost_enable_notify(), I'm unable
to hit the issue. I will try for more times to make sure the issue is really resolved.
After that, I will post formal patches for review.

Thanks,
Gavin