linux-kernel - Re: [PATCH 1/4] cpu_stop: implement stop

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Message-ID: <4BDFC6C0.7020009@kernel.org>
Date:	Tue, 04 May 2010 09:03:28 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	mingo@...e.hu, linux-kernel@...r.kernel.org, x86@...nel.org,
	oleg@...hat.com, rusty@...tcorp.com.au, sivanich@....com,
	heiko.carstens@...ibm.com, dipankar@...ibm.com,
	josh@...edesktop.org, paulmck@...ux.vnet.ibm.com,
	akpm@...ux-foundation.org, arjan@...ux.intel.com,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH 1/4] cpu_stop: implement stop_cpu[s]()

On 05/04/2010 08:36 AM, Tejun Heo wrote:
> On 05/03/2010 03:26 PM, Peter Zijlstra wrote:
>> On Thu, 2010-04-22 at 18:09 +0200, Tejun Heo wrote:
>>> +static int cpu_stopper_thread(void *data)
>>> +{
>>> +       struct cpu_stopper *stopper = data; 
>>
>> BUG_ON(stopper != __get_cpu_var(cpu_stopper)); ?
> 
> Added.

Now that I think more about it, there's a subtle race condition with
the above BUG_ON().  Stoppers are prepared by CPU_UP_PREPARE and
started by CPU_ONLINE but brought down by CPU_DEAD.  IOW, they're
allowed to run detached from their designated CPUs between CPU_DYING
and CPU_DEAD (the reponsibility of guaranteeing target cpus's onliness
is on the callers).  So, the above BUG_ON() might trigger spuriously
if a cpu goes down after brought online before its cpu_stopper had a
chance to pass through the BUG_ON() test.

Thanks.

-- 
tejun
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