Re: [V9fs-developer] [PATCH v2] net/9p: Fix a deadlock case in the virtio transport

[jiangyiwen]

On 2018/7/17 19:42, Dominique Martinet wrote:
> 
>> Subject: net/9p: Fix a deadlock case in the virtio transport
> 
> I hadn't noticed in the v1, but how is that a deadlock fix?
> The previous code doesn't look like it deadlocks to me, the commit
> message is more correct.
> 

Hi Dominique,

If cpu is running in the irq context for a long time,
NMI watchdog will detect the hard lockup in the cpu,
and then it will cause kernel panic. So I use this
subject to underline the scenario.

> jiangyiwen wrote on Tue, Jul 17, 2018:
>> When client has multiple threads that issue io requests
>> all the time, and the server has a very good performance,
>> it may cause cpu is running in the irq context for a long
>> time because it can check virtqueue has buf in the *while*
>> loop.
>>
>> So we should keep chan->lock in the whole loop.
>>
>> Signed-off-by: Yiwen Jiang <jiangyiwen@xxxxxxxxxx>
>> ---
>>  net/9p/trans_virtio.c | 17 ++++++-----------
>>  1 file changed, 6 insertions(+), 11 deletions(-)
>>
>> diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
>> index 05006cb..e5fea8b 100644
>> --- a/net/9p/trans_virtio.c
>> +++ b/net/9p/trans_virtio.c
>> @@ -148,20 +148,15 @@ static void req_done(struct virtqueue *vq)
>>
>>  	p9_debug(P9_DEBUG_TRANS, ": request done\n");
>>
>> -	while (1) {
>> -		spin_lock_irqsave(&chan->lock, flags);
>> -		req = virtqueue_get_buf(chan->vq, &len);
>> -		if (req == NULL) {
>> -			spin_unlock_irqrestore(&chan->lock, flags);
>> -			break;
>> -		}
>> -		chan->ring_bufs_avail = 1;
>> -		spin_unlock_irqrestore(&chan->lock, flags);
>> -		/* Wakeup if anyone waiting for VirtIO ring space. */
>> -		wake_up(chan->vc_wq);
>> +	spin_lock_irqsave(&chan->lock, flags);
>> +	while ((req = virtqueue_get_buf(chan->vq, &len)) != NULL) {
>>  		if (len)
>>  			p9_client_cb(chan->client, req, REQ_STATUS_RCVD);
>>  	}
>> +	chan->ring_bufs_avail = 1;
> 
> Do we have a guarantee that req_done is only called if there is at least
> one buf to read?
> For example, that there isn't two threads queueing the same callback but
> the first one reads everything and the second has nothing to read?
> 
> If virtblk_done takes care of setting up a "req_done" bool to only
> notify waiters if something has been done I'd rather have a reason to do
> differently, even if you can argue that nothing bad will happen in case
> of a gratuitous wake_up
> 

Sorry, I don't fully understand what your mean.
I think even if the ring buffer don't have the data, wakeup operation
will not cause any other problem, and the loss of performance can be
ignored.

Thanks.

>> +	spin_unlock_irqrestore(&chan->lock, flags);
>> +	/* Wakeup if anyone waiting for VirtIO ring space. */
>> +	wake_up(chan->vc_wq);
>>  }
> 
> Thanks,
>