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

From: Laura Abbott <redacted>
Date: Mon, 14 Mar 2016 13:07:06 -0700

On 03/14/2016 12:24 PM, David Miller wrote:

quoted

From: Claudio Imbrenda <redacted>
Date: Fri, 11 Mar 2016 13:39:23 +0100

quoted

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.

Ok, can you please submit this anew?  Your commit message format was
incorrect, you put the commit message content you wanted in the change
after the --- separater instead of beforehand.