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

Eric Dumazet
2013-01-01T18:42:36Z

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.