linux-kernel - Re: sched: hang in migrate

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Date:	Wed, 14 May 2014 14:21:04 +0400
From:	Kirill Tkhai <tkhai@...dex.ru>
To:	Peter Zijlstra <peterz@...radead.org>
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



14.05.2014, 14:14, "Peter Zijlstra" <peterz@...radead.org>:
> 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.

It's about stop_two_cpus(), I have a distrust about other users of stop task:

queue_stop_cpus_work() queues work consequentially:

0 1 2 4

stop_two_cpus() may queue:

1 0

Looks like, stop thread on 0th and on 1th are waiting for wrong works.
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