Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [V9fs-developer] [PATCH v2] net/9p: Fix a deadlock case in the
 virtio transport
To:     Dominique Martinet <asmadeus@codewreck.org>
References: <5B4DCD0A.8040600@huawei.com> <20180717114215.GA14414@nautica>
CC:     Andrew Morton <akpm@linux-foundation.org>,
        Eric Van Hensbergen <ericvh@gmail.com>,
        Ron Minnich <rminnich@sandia.gov>,
        Latchesar Ionkov <lucho@ionkov.net>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        <v9fs-developer@lists.sourceforge.net>
From:   jiangyiwen <jiangyiwen@huawei.com>
Message-ID: <5B4DE09F.5000800@huawei.com>
Date:   Tue, 17 Jul 2018 20:27:11 +0800
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101
 Thunderbird/38.5.1
MIME-Version: 1.0
In-Reply-To: <20180717114215.GA14414@nautica>
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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@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
> 

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,
>