Re: [PATCHv2] vsock: Fix blocking ops call in prepare_to_wait
From: David Miller
Date: Mon Mar 14 2016 - 15:24:11 EST
From: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxxxxxxx>
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
Can someone please look at this? Who maintains this code anyways?