linux-kernel - Re: smp_call_function_single with wait=0 considered harmful

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Date:	Fri, 28 Feb 2014 12:34:27 -0500
From:	Prarit Bhargava <prarit@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tony Luck <tony.luck@...el.com>,
	Robert Richter <rric@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Aaro Koskinen <aaro.koskinen@....fi>,
	David Daney <david.daney@...ium.com>,
	linux-kernel@...r.kernel.org, Rik van Riel <riel@...hat.com>,
	Mel Gorman <mgorman@...e.de>
Subject: Re: smp_call_function_single with wait=0 considered harmful



On 02/28/2014 07:39 AM, Peter Zijlstra wrote:
> On Fri, Feb 28, 2014 at 01:26:24PM +0100, Peter Zijlstra wrote:
>> On Wed, Dec 04, 2013 at 08:46:27AM -0800, Christoph Hellwig wrote:
>>>   kernel/stop_machine.c:stop_two_cpus()
>>
>> That site should work with .wait=1 just fine, but given the above, the
>> .wait=0 doesn't appear problematic at all.
> 
> Scratch that; its broken, but not because of smp_call_function_single().
> 
> ---
> Subject: stop_machine: Fix^2 race between stop_two_cpus() and stop_cpus()
> 
> We must use smp_call_function_single(.wait=1) for the
> irq_cpu_stop_queue_work() to ensure the queueing is actually done under
> stop_cpus_lock. Without this we could have dropped the lock by the time
> we do the queueing and get the race we tried to fix.
> 
> Fixes: 7053ea1a34fa ("stop_machine: Fix race between stop_two_cpus() and stop_cpus()")
> Cc: Prarit Bhargava <prarit@...hat.com>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Mel Gorman <mgorman@...e.de>
> Signed-off-by: Peter Zijlstra <peterz@...radead.org>

Reviewed-by: Prarit Bhargava <prarit@...hat.com>

P.
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