linux-kernel - Re: [PATCH v2 1/2] epoll: introduce POLLFREE to flush ->signalfd

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Date:	Thu, 1 Mar 2012 20:26:36 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Lutomirski <luto@....edu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Davide Libenzi <davidel@...ilserver.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Greg KH <greg@...ah.com>, Jason Baron <jbaron@...hat.com>,
	Roland McGrath <roland@...k.frob.com>,
	Eugene Teo <eugeneteo@...nel.sg>,
	Maxime Bizon <mbizon@...ebox.fr>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] epoll: introduce POLLFREE to flush
	->signalfd_wqh before kfree()

On 02/29, Andrew Lutomirski wrote:
>
> On Wed, Feb 29, 2012 at 12:06 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> > On 02/29, Andy Lutomirski wrote:
> >>
> >> On 02/24/2012 11:07 AM, Oleg Nesterov wrote:
> >> > This patch adds the special event, POLLFREE, currently only for
> >> > epoll. It expects that init_poll_funcptr()'ed hook should do the
> >> > necessary cleanup. Perhaps it should be defined as EPOLLFREE in
> >> > eventpoll.
> >>
> >> I have a bunch of userspace code that uses signalfd via epoll.  Does
> >> this affect the ABI?
> >
> > I hope not ;)
> >
>
> Excellent!
>
> To avoid further confusion, would it make sense to update the comment
> in poll.h to indicate that POLLFREE is only for epoll and is internal
> to the kernel?

Agreed. Or perhaps we can simply use POLLREMOVE instead...

But note that nobody (including me) likes this fix, just I was asked
to make something simple/backportable. Probably it will be reverted
later.

If not, then perhaps it makes sense to do a couple of simple changes
on top to cleanup the usage of POLLFREE and to ensure that POLLHUP
is actually delivered (the latter will be user-visible).

Oleg.

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