Message-ID: <493E89AD.8000106@linux.vnet.ibm.com>
Date: Tue, 09 Dec 2008 09:07:25 -0600
From: Brian King <brking@linux.vnet.ibm.com>
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To: Peter Zijlstra <a.p.zijlstra@chello.nl>
CC: mingo@elte.hu, efault@gmx.de, vatsa@in.ibm.com, balbir@linux.vnet.ibm.com,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] sched: CPU remove deadlock fix
References: <200812091548.mB9Fm0sm011959@d01av04.pok.ibm.com> <1228834342.8684.5.camel@twins>
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Peter Zijlstra wrote:
> On Tue, 2008-12-09 at 08:47 -0600, Brian King wrote:
>> This patch fixes a possible deadlock scenario in the CPU remove path.
>> migration_call grabs rq->lock, then wakes up everything on rq->migration_queue
>> with the lock held. Then one of the tasks on the migration queue ends up
>> calling tg_shares_up which then also tries to acquire the same rq->lock.
> 
> Looks ok, does lockdep agree?

I didn't have lockdep enabled when I verified it, but the section of code now
looks like:

spin_lock_irq(&rq->lock);
while (!list_empty(&rq->migration_queue)) {
	struct migration_req *req;

	req = list_entry(rq->migration_queue.next,
		 struct migration_req, list);
	list_del_init(&req->list);
	spin_unlock_irq(&rq->lock);
	complete(&req->done);
	spin_lock_irq(&rq->lock);
}
spin_unlock_irq(&rq->lock);

So I'm pretty sure lockdep will agree.

-Brian

> 
>> Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
>> ---
>>
>>  kernel/sched.c |    2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff -puN kernel/sched.c~sched_cpu_down_deadlock_fix kernel/sched.c
>> --- linux-2.6/kernel/sched.c~sched_cpu_down_deadlock_fix	2008-12-09 08:42:09.000000000 -0600
>> +++ linux-2.6-bjking1/kernel/sched.c	2008-12-09 08:42:09.000000000 -0600
>> @@ -6587,7 +6587,9 @@ migration_call(struct notifier_block *nf
>>  			req = list_entry(rq->migration_queue.next,
>>  					 struct migration_req, list);
>>  			list_del_init(&req->list);
>> +			spin_unlock_irq(&rq->lock);
>>  			complete(&req->done);
>> +			spin_lock_irq(&rq->lock);
>>  		}
>>  		spin_unlock_irq(&rq->lock);
>>  		break;
>> _
> 


-- 
Brian King
Linux on Power Virtualization
IBM Linux Technology Center


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