linux-kernel - Re: sched: hang in migrate

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Date:	Wed, 14 May 2014 12:13:54 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Kirill Tkhai <tkhai@...dex.ru>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	Michael wang <wangyun@...ux.vnet.ibm.com>,
	"ktkhai@...allels.com" <ktkhai@...allels.com>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: sched: hang in migrate_swap

On Wed, May 14, 2014 at 01:42:32PM +0400, Kirill Tkhai wrote:
> Peter, do we have to queue stop works orderly?
> 
> Is there is not a possibility, when two pair of works queued different on
> different cpus?
> 
> 
>  kernel/stop_machine.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> index b6b67ec..29e221b 100644
> --- a/kernel/stop_machine.c
> +++ b/kernel/stop_machine.c
> @@ -250,8 +250,14 @@ struct irq_cpu_stop_queue_work_info {
>  static void irq_cpu_stop_queue_work(void *arg)
>  {
>  	struct irq_cpu_stop_queue_work_info *info = arg;
> -	cpu_stop_queue_work(info->cpu1, info->work1);
> -	cpu_stop_queue_work(info->cpu2, info->work2);
> +
> +	if (info->cpu1 < info->cpu2) {
> +		cpu_stop_queue_work(info->cpu1, info->work1);
> +		cpu_stop_queue_work(info->cpu2, info->work2);
> +	} else {
> +		cpu_stop_queue_work(info->cpu2, info->work2);
> +		cpu_stop_queue_work(info->cpu1, info->work1);
> +	}
>  }

I'm not sure, we already send the IPI to the first cpu of the pair, so
supposing we have 4 cpus, and get 4 pairs like:

0,1 1,2 2,3 3,0

That would result in IPIs to 0, 1, 2, and 0 again, and since the IPI
function is serialized I don't immediately see a way for this to
deadlock.



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