From mboxrd@z Thu Jan  1 00:00:00 1970
From: Eric Wong <normalperson@yhbt.net>
Subject: Re: [PATCH] poll: prevent missed events if _qproc is NULL
Date: Tue, 1 Jan 2013 21:17:28 +0000
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Cc: linux-kernel@vger.kernel.org,
	Hans Verkuil <hans.verkuil@cisco.com>,
	Jiri Olsa <jolsa@redhat.com>, Jonathan Corbet <corbet@lwn.net>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Davide Libenzi <davidel@xmailserver.org>,
	Hans de Goede <hdegoede@redhat.com>,
	Mauro Carvalho Chehab <mchehab@infradead.org>,
	David Miller <davem@davemloft.net>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andreas Voellmy <andreas.voellmy@yale.edu>,
	"Junchang(Jason) Wang" <junchang.wang@yale.edu>,
	netdev@vger.kernel.org, linux-fsdevel@vger.kernel.org
To: Eric Dumazet <eric.dumazet@gmail.com>
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Eric Wong <normalperson@yhbt.net> wrote:
> Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > commit 626cf236608505d376e4799adb4f7eb00a8594af should not have this
> > side effect, at least for poll()/select() functions. The epoll() changes
> > I am not yet very confident.
> 
> I have a better explanation of the epoll problem below.
> 
> An alternate version (limited to epoll) would be:
> 
> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> index cd96649..ca5f3d0 100644
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -1299,6 +1299,7 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_even
>  	 * Get current event bits. We can safely use the file* here because
>  	 * its usage count has been increased by the caller of this function.
>  	 */
> +	smp_mb();
>  	revents = epi->ffd.file->f_op->poll(epi->ffd.file, &pt);
>  
>  	/*
> 
> > I suspect a race already existed before this commit, it would be nice to
> > track it properly.
> 
> I don't believe this race existed before that change.

I was wrong, rereading 626cf236608505d376e4799adb4f7eb00a8594af,
I think this race existed before.

Perhaps my alternate patch above is a better fix.