Subject: Re: [PATCH 1/1] sched: CPU remove deadlock fix
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Brian King <brking@linux.vnet.ibm.com>
Cc: mingo@elte.hu, efault@gmx.de, vatsa@in.ibm.com, balbir@linux.vnet.ibm.com,
       linux-kernel@vger.kernel.org
In-Reply-To: <200812091548.mB9Fm0sm011959@d01av04.pok.ibm.com>
References: <200812091548.mB9Fm0sm011959@d01av04.pok.ibm.com>
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Date: Tue, 09 Dec 2008 15:52:22 +0100
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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?

> 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;
> _

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