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Subject: Re: [PATCH] Make sure timers have migrated before killing
 migration_thread
From: Peter Zijlstra <peterz@infradead.org>
To: "Amit K. Arora" <aarora@linux.vnet.ibm.com>
Cc: tj@kernel.org, Ingo Molnar <mingo@elte.hu>,
       Srivatsa Vaddagiri <vatsa@in.ibm.com>,
       Gautham R Shenoy <ego@in.ibm.com>, Darren Hart <dvhltc@us.ibm.com>,
       Brian King <brking@linux.vnet.ibm.com>, linux-kernel@vger.kernel.org
In-Reply-To: <20100524095951.GA17680@amitarora.in.ibm.com>
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Date: Tue, 25 May 2010 13:31:35 +0200
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On Mon, 2010-05-24 at 15:29 +0530, Amit K. Arora wrote:
> 
> Thus, since above race can never happen, is there any other issue with
> this patch ?

It doesn't seem to apply nicely...

> 
> Signed-off-by: Amit Arora <aarora@in.ibm.com>
> Signed-off-by: Gautham R Shenoy <ego@in.ibm.com>
> ---
> diff -Nuarp linux-2.6.34.org/kernel/sched.c linux-2.6.34/kernel/sched.c
> --- linux-2.6.34.org/kernel/sched.c     2010-05-18 22:56:21.000000000 -0700
> +++ linux-2.6.34/kernel/sched.c 2010-05-18 22:58:31.000000000 -0700
> @@ -5942,14 +5942,26 @@ migration_call(struct notifier_block *nf
>                 cpu_rq(cpu)->migration_thread = NULL;
>                 break;
>  
> +       case CPU_POST_DEAD:
> +               /*
> +                * Bring the migration thread down in CPU_POST_DEAD event,
> +                * since the timers should have got migrated by now and thus
> +                * we should not see a deadlock between trying to kill the
> +                * migration thread and the sched_rt_period_timer.
> +                */
> +               cpuset_lock();
> +               rq = cpu_rq(cpu);
> +               kthread_stop(rq->migration_thread);
> +               put_task_struct(rq->migration_thread);
> +               rq->migration_thread = NULL;
> +               cpuset_unlock();
> +               break;
> +
>         case CPU_DEAD:
>         case CPU_DEAD_FROZEN:
>                 cpuset_lock(); /* around calls to cpuset_cpus_allowed_lock() */
>                 migrate_live_tasks(cpu);
>                 rq = cpu_rq(cpu);
> -               kthread_stop(rq->migration_thread);
> -               put_task_struct(rq->migration_thread);
> -               rq->migration_thread = NULL;
>                 /* Idle task back to normal (off runqueue, low prio) */
>                 raw_spin_lock_irq(&rq->lock);
>                 update_rq_clock(rq);
> 
>