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Date: Tue, 01 Jan 2013 10:42:30 -0800
From: Eric Dumazet <eric.dumazet@...il.com>
To: Eric Wong <normalperson@...t.net>
Cc: linux-kernel@...r.kernel.org,
Hans Verkuil <hans.verkuil@...co.com>,
Jiri Olsa <jolsa@...hat.com>, Jonathan Corbet <corbet@....net>,
Al Viro <viro@...iv.linux.org.uk>,
Davide Libenzi <davidel@...ilserver.org>,
Hans de Goede <hdegoede@...hat.com>,
Mauro Carvalho Chehab <mchehab@...radead.org>,
David Miller <davem@...emloft.net>,
Andrew Morton <akpm@...ux-foundation.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Andreas Voellmy <andreas.voellmy@...e.edu>,
"Junchang(Jason) Wang" <junchang.wang@...e.edu>,
netdev@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] poll: prevent missed events if _qproc is NULL
On Mon, 2012-12-31 at 13:21 +0000, Eric Wong 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
>
> 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/
Hmm, the change seems not very logical to me.
If it helps, I would like to understand the real issue.
commit 626cf236608505d376e4799adb4f7eb00a8594af should not have this
side effect, at least for poll()/select() functions. The epoll() changes
I am not yet very confident.
I suspect a race already existed before this commit, it would be nice to
track it properly.
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