Re: [PATCH] poll: prevent missed events if _qproc is NULL

Junchang(Jason) Wang
2013-01-01T16:58:17Z

Hi Eric and list,

Thanks a lot. The patch solves our (Andreas and my) issue in using
epoll. Here's our test program
https://github.com/AndreasVoellmy/epollbug/blob/master/epollbug.c We
are using Linux 3.7.1 and a server with 80 cores.

Cheers!

--Jason

On Mon, Dec 31, 2012 at 6:24 PM, Eric Wong <normalperson@yhbt.net> wrote:
>
> Eric Wong <normalperson@yhbt.net> wrote:
> > This patch seems to fix my issue with ppoll() being stuck on my
> > SMP machine: http://article.gmane.org/gmane.linux.file-systems/70414
>
> OK, it doesn't fix my issue, but it seems to make it harder-to-hit...
>
> > The change to sock_poll_wait() in
> > commit 626cf236608505d376e4799adb4f7eb00a8594af
> > (poll: add poll_requested_events() and poll_does_not_wait() functions)
> > seems to have allowed additional cases where the SMP memory barrier
> > is not issued before checking for readiness.
> >
> > In my case, this affects the select()-family of functions
> > which register descriptors once and set _qproc to NULL before
> > checking events again (after poll_schedule_timeout() returns).
> > The set_mb() barrier in poll_schedule_timeout() appears to be
> > insufficient on my SMP x86-64 machine (as it's only an xchg()).
> >
> > This may also be related to the epoll issue described by
> > Andreas Voellmy in http://thread.gmane.org/gmane.linux.kernel/1408782/
>
> However, I believe my patch will still fix Andreas' issue with epoll
> due to how ep_modify() uses a NULL qproc when calling ->poll().
>
> (I've never been able to reproduce Andreas' issue on my 4-core system,
> but he's been hitting it since 3.4 (at least))