From: Claudio Imbrenda <imbre...@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?
