linux-kernel - Re: [PATCH] CPU hotplug, smpboot: Fix crash in smpboot_thread

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Date:	Thu, 11 Apr 2013 10:10:54 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
cc:	Dave Hansen <dave@...1.net>, Borislav Petkov <bp@...en8.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>, dhillf@...il.com,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] CPU hotplug, smpboot: Fix crash in smpboot_thread_fn()

On Wed, 10 Apr 2013, Srivatsa S. Bhat wrote:

> Interestingly, in every single stack trace, the crashing task is the migration
> thread. Now, migration thread belongs to the highest priority stop_task sched
> class, and this particular sched class is very unique in the way it implements
> its internal sched class functions, and I suspect this has a lot of bearing
> on how functions like kthread_bind(), wake_up_process() etc react with it
> (by looking at how it implements its functions such as select_task_rq(),
> enqueue_task(), dequeue_task() etc).

I don't think that's relevant. The migration thread can only be woken
via try_to_wakeup and my previous patch which implements a separate
task state makes sure that it cannot be woken accidentaly by anything
else than unpark.
 
> But note that __kthread_bind() can wake up the task if the task is an RT
> task. So it can be called only when the CPU (to which we want to bind the task)

kthread_bind() does NOT wakeup anything. It merily sets the cpus
allowed ptr without further ado.


Thanks,

	tglx
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