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

From: Gavin Shan <gshan@redhat.com>
To: Will Deacon <will@kernel.org>, Keir Fraser <keirf@google.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>,
	virtualization@lists.linux.dev, linux-kernel@vger.kernel.org,
	jasowang@redhat.com, xuanzhuo@linux.alibaba.com,
	yihyu@redhat.com, shan.gavin@gmail.com,
	linux-arm-kernel@lists.infradead.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	mochs@nvidia.com
Subject: Re: [PATCH] virtio_ring: Fix the stale index in available ring
Date: Wed, 27 Mar 2024 10:01:35 +1000	[thread overview]
Message-ID: <50d6fec8-708b-4174-b6d4-388083312bb8@redhat.com> (raw)
In-Reply-To: <6bdf2884-852e-42d3-9e67-c9d5aa7d932a@redhat.com>

On 3/27/24 09:14, Gavin Shan wrote:
> 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 again, Will. The formal patches have been sent for review.

https://lkml.org/lkml/2024/3/27/40

Thanks,
Gavin