linux-kernel - Re: Polling(2) /proc/mounts is broken?

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Date:	Mon, 13 Dec 2010 12:26:18 +0000
From:	Tvrtko Ursulin <tvrtko.ursulin@...hos.com>
To:	Eric Paris <eparis@...hat.com>
CC:	Al Viro <viro@...iv.linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Polling(2) /proc/mounts is broken?

On Friday 10 Dec 2010 15:32:48 Tvrtko Ursulin wrote:
> Hi all,
>
> So since we cannot get fanotify global mode we were told to poll(2) on
> /proc/mounts and set up new mount marks dynamically.
>
> Is the attached C program roughly how it should work?
>
> Because on 2.6.37-rc5 it busy loops for me (POLLIN always reported), even
> though if I remove the seek it gets EOF immediately after POLLIN.
>
> Under 2.6.32 (Ubuntu flavour) I see the same behaviour.
>
> Going further back to 2.6.24 (again Ubuntu flavour) and there it works, but
> it set POLLERR instead of POLLIN in revents.
>
> So I guess it regressed somewhere in between. I'll look to provide a fix
> unless someone more familiar with this area beats me to it.

It hasn't regressed but was fixed in 2.6.30. :)

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=31b07093c44a7a442394d44423e21d783f5523b8

So it is incorrect to poll for POLLIN. In case of /proc/mounts POLLPRI should
be used and will be returned together with POLLERR.

Tvrtko

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