Re: [PATCHv2] vsock: Fix blocking ops call in prepare_to_wait

[David Miller <davem@davemloft.net>]

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?