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Subject: Re: [V9fs-developer] [PATCH] net/9p: Fix a deadlock case in the
 virtio transport
To:     Dominique Martinet <asmadeus@codewreck.org>
References: <5B49B8CF.40709@huawei.com> <20180714090502.GA16186@nautica>
 <5B49DAA5.3020600@huawei.com> <20180714124715.GA16134@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: <5B4BFB29.3080507@huawei.com>
Date:   Mon, 16 Jul 2018 09:55:53 +0800
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On 2018/7/14 20:47, Dominique Martinet wrote:
> jiangyiwen wrote on Sat, Jul 14, 2018:
>> 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.
> 
> Fair enough. If you do have some numbers to give though (throughput
> and/or iops before/after) I'd still be really curious.
> 
>>>>  		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.
> 
> Please do, you listed virtio_blk as doing this and they have the same
> kind of pattern with a req_done bool and only restarting stopped queues
> if they processed something
> 

You're right, this wake up operation should be put after the unlocking,
I will resend it. In addition, whether I should resend this patch based
on your 9p-next branch?

Thanks,
Yiwen.