linux-kernel - Re: [PATCH 6/8] blackfin: Fix possible race on task->mm

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Date:	Wed, 8 Feb 2012 17:20:13 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Anton Vorontsov <anton.vorontsov@...aro.org>
Cc:	Greg KH <gregkh@...uxfoundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	Russell King <linux@....linux.org.uk>,
	Mike Frysinger <vapier@...too.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Richard Weinberger <richard@....at>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/8] blackfin: Fix possible race on task->mm

On 02/07, Anton Vorontsov wrote:
>
> Even when in atomic context, grabbing irqsave variant of tasklist lock
> is not enough to protect task->mm from disappearing on SMP machines.
> Instead, we have to grab the task lock.

Yes, but afaics there is no reason for write_lock_irqsave(tasklist).
read_lock() should be enough.

I know nothing about arch/blackfin/ but in fact this looks simply wrong.

For example. sysrq_showregs_othercpus() does smp_call_function(showacpu)
and showacpu() show_stack()->decode_address(). Now suppose that IPI
interrupts the task holding read_lock(tasklist).

Mike?

Oleg.

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