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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>
 <5B4DE09F.5000800@huawei.com> <20180717130720.GA23759@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: <5B4E90BE.5030307@huawei.com>
Date:   Wed, 18 Jul 2018 08:58:38 +0800
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On 2018/7/17 21:07, Dominique Martinet wrote:
> jiangyiwen wrote on Tue, Jul 17, 2018:
>> 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.
>>>
>>
>> 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.
> 
> That's still not a deadlock - fix lockup would be more appropriate?
> 
> 

Okay.

>>> 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.
> 
> I just mean "others do check, why not us?". It's almost free to check if
> we had something to read, but if there are many pending read/writes
> waiting for a buffer they will all wake up and spin uselessly.
> 
> I've checked other callers of virtqueue_get_buf() and out of 9 that loop
> around in a callback then wake another thread up, 6 do check before
> waking up, two check that something happened just to print a debug
> statement if not (virtio_test and virtgpu) and one doesn't check
> (virtio_input); so I guess we wouldn't be the first ones, just not
> following the trend.
> 
> But yes, nothing bad will happen, so let's agree to disagree and I'll
> defer to others opinion on this
> 
> 
> Thanks,
> 

Thanks for your reply, you're right, other callers also check whether
Virtio ring has data then do wakeup operation, we also should follow
the trend.

Okay, I will resend the patch later.

Thanks,
Yiwen.