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

[jiangyiwen <jiangyiwen@huawei.com>]

On 2018/7/14 17:05, Dominique Martinet wrote:
> jiangyiwen wrote on Sat, Jul 14, 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.
> 
> Hmm, this is generally bad practice to hold a spin lock for long.
> In general, spin locks are meant to protect data, not code.
> 
> I'd want some numbers to decide on this one, even if I think this
> particular case is safe (e.g. this cannot dead-lock)
> 

Actually, the loop will not hold a spin lock for long, because other
threads will not issue new requests in this case. In addition,
virtio-blk or virtio-scsi also use this solution, I guess it may also
encounter this problem before.

>> Signed-off-by: Yiwen Jiang <jiangyiwen@huawei.com>
>> ---
>>  net/9p/trans_virtio.c | 8 +++-----
>>  1 file changed, 3 insertions(+), 5 deletions(-)
>>
>> diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
>> index 05006cb..9b0f5f2 100644
>> --- a/net/9p/trans_virtio.c
>> +++ b/net/9p/trans_virtio.c
>> @@ -148,20 +148,18 @@ static void req_done(struct virtqueue *vq)
>>
>>  	p9_debug(P9_DEBUG_TRANS, ": request done\n");
>>
>> +	spin_lock_irqsave(&chan->lock, flags);
>>  	while (1) {
>> -		spin_lock_irqsave(&chan->lock, flags);
>>  		req = virtqueue_get_buf(chan->vq, &len);
>> -		if (req == NULL) {
>> -			spin_unlock_irqrestore(&chan->lock, flags);
>> +		if (req == NULL)
>>  			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);
> 
> In particular, the wake up here echoes to wait events that will
> immediately try to grab the lock, and will needlessly spin on it until
> this thread is done.
> If we do go this way I'd want setting chan->ring_bufs_avail to be done
> just before unlocking and the wakeup to be done just after unlocking out
> of the loop iff we processed at least one iteration here.
> 

I can move the wakeup operation after the unlocking. Like what I said
above, I think this loop will not execute for long.

Thanks,
Yiwen.

> That should also save you precious cpu cycles while under lock :)
>