lkml.org 
[lkml]   [2012]   [Dec]   [31]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] poll: prevent missed events if _qproc is NULL
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))


\
 
 \ /
  Last update: 2013-01-01 00:41    [W:0.155 / U:1.012 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site