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

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Date:	Wed, 29 Feb 2012 21:06:36 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andy 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, 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 ;)

> Will epoll_wait ever set POLLFREE?

No.

> Does
> EPOLL_CTL_MOD accept POLLFREE?

Yes, it doesn't check the bits. EPOLL_CTL_MOD accepts any bit.

> IOW, from a userspace point of view, wtf does this do?

I hope this is transparent to the user-space. If the application
does EPOLL_CTL_MOD(POLLFREE) by mistake then ep_poll_callback(POLLFREE)
can trigger the unnecessary wakeup and move the file to ep->rdllist
but this is harmless. The subsequent ->poll() can't return POLLFREE.

Oleg.

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