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

Eric Wong [off-list ref] 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))