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

[Dominique Martinet <asmadeus@codewreck.org>]

> 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.

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@huawei.com>
> ---
>  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

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

Thanks,
-- 
Dominique