linux-kernel - Re: epoll oops.

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Message-ID: <20131016223922.GA18383@dcvr.yhbt.net>
Date:	Wed, 16 Oct 2013 22:39:22 +0000
From:	Eric Wong <normalperson@...t.net>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Davide Libenzi <davidel@...ilserver.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Peter Hurley <peter@...leysoftware.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: epoll oops.

Oleg Nesterov <oleg@...hat.com> wrote:
> Yes. Before that 971316f0503a hack epoll can't even know if the task
> which did signalfd_poll() exits and frees the active signalfd_wqh.
> If for example that task forked a child before exit.
> 
> And the whole RCU logic is only needed if exit/ep_remove_wait_queue
> actually race with each other.

Is there any chance this oops is caused by (or at least more easily
exposed by) commit 91cf5ab60ff82ecf4550a596867787c1e360dd3f ?
(epoll: add a reschedule point in ep_free())

I thought 91cf5ab would be benign, except...

> Yes, ugly, agreed. d80e731ecab4 even tries to docunent that this all
> is the hack.

.. the following sentence from d80e731ecab4 caught my eye:

    It also assumes that nobody can take tasklist_lock under epoll
    locks, this seems to be true.

I haven't been able to trace if cond_resched() can take tasklist_lock.
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