linux-kernel - Re: [PATCH RFC] sched: Revert delayed_put_task

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Message-ID: <20141017213440.GA32576@redhat.com>
Date:	Fri, 17 Oct 2014 23:34:40 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Kirill Tkhai <tkhai@...dex.ru>
Cc:	Kirill Tkhai <ktkhai@...allels.com>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Vladimir Davydov <vdavydov@...allels.com>
Subject: Re: [PATCH RFC] sched: Revert delayed_put_task_struct() and fix
	use after free

On 10/16, Kirill Tkhai wrote:
>
> Cool! Elegant fix. We set PF_EXITING in exit_signals(), which is earlier
> than release_task() is called.

OK, thanks, I am sending the patch...

> Shouldn't we use smp_rmb/smp_wmb here?

No, we do not. call_rcu(delayed_put_pid) itself implies the barrier on
all CPUs. IOW, by the time RCU actually calls delayed_put_pid() every
CPU must see all memory changes which were done before call_rcu() was
called. And otoh, all rcu-read-lock critical sections which could miss
PF_EXITING should be already finished.

Oleg.

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