Subject: Re: [PATCHv2] vsock: Fix blocking ops call in prepare_to_wait
To: David Miller <davem@davemloft.net>, imbrenda@linux.vnet.ibm.com
References: <1454611845-5358-1-git-send-email-labbott@fedoraproject.org>
 <20160213.055940.1894427692019983187.davem@davemloft.net>
 <1457699963-27694-1-git-send-email-imbrenda@linux.vnet.ibm.com>
 <20160314.152403.1490973796884839758.davem@davemloft.net>
Cc: linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        labbott@fedoraproject.org
From: Laura Abbott <labbott@redhat.com>
Message-ID: <56E719EA.7040208@redhat.com>
Date: Mon, 14 Mar 2016 13:07:06 -0700
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On 03/14/2016 12:24 PM, David Miller wrote:
> From: Claudio Imbrenda <imbrenda@linux.vnet.ibm.com>
> Date: Fri, 11 Mar 2016 13:39:23 +0100
>
>> I think I found a problem with the patch submitted by Laura Abbott
>> ( https://lkml.org/lkml/2016/2/4/711 ): we might miss wakeups.
>> Since the condition is not checked between the prepare_to_wait and the
>> schedule(), if a wakeup happens after the condition is checked but before
>> the sleep happens, and we miss it. ( A description of the problem can be
>> found here: http://www.makelinux.net/ldd3/chp-6-sect-2 ).
>>
>> My solution (see patch below) is to shrink the area influenced by
>> prepare_to_wait, but keeping the fragile section around the condition, and
>> keep the rest of the code in "normal" running state.  This way the sleep is
>> correct and the other functions don't need to worry.  The only caveat here
>> is that the function(s) called to verify the conditions are really not
>> allowed to sleep, so if you need synchronization in the backend of e.g.
>> vsock_stream_has_space(), you should use spinlocks and not mutexes.
>>
>> In case we want to be able to sleep while waiting for conditions, we can
>> consider this instead: https://lwn.net/Articles/628628/ .
>>
>>
>> I stumbled on this problem while working on fixing the upcoming virtio
>> backend for vsock, below is the patch I had prepared, with the original
>> message.
>
> Can someone please look at this?  Who maintains this code anyways?
>

Nobody was listed in MAINTAINERS. I tried cc-ing some of the e-mail addresses
of the original authors (vmware?) when sending the original patch and they
all bounced.

Thanks,
Laura