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Date: Fri, 28 Feb 2014 12:34:27 -0500
From: Prarit Bhargava <prarit@redhat.com>
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To: Peter Zijlstra <peterz@infradead.org>
CC: Christoph Hellwig <hch@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
        Tony Luck <tony.luck@intel.com>, Robert Richter <rric@kernel.org>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Aaro Koskinen <aaro.koskinen@iki.fi>,
        David Daney <david.daney@cavium.com>, linux-kernel@vger.kernel.org,
        Rik van Riel <riel@redhat.com>, Mel Gorman <mgorman@suse.de>
Subject: Re: smp_call_function_single with wait=0 considered harmful
References: <20131204164627.GA27677@infradead.org> <20140228122624.GF9987@twins.programming.kicks-ass.net> <20140228123905.GK3104@twins.programming.kicks-ass.net>
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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@redhat.com>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Signed-off-by: Peter Zijlstra <peterz@infradead.org>

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

P.