linux-kernel - Re: [PATCH 1/2] hrtimer: Consider preemption when migrating hrtimer cpu

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Date:	Thu, 18 Apr 2013 11:40:32 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Michael Bohan <mbohan@...eaurora.org>
cc:	LKML <linux-kernel@...r.kernel.org>, linux-arm-msm@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 1/2] hrtimer: Consider preemption when migrating hrtimer
 cpu_bases

On Wed, 10 Apr 2013, Michael Bohan wrote:

> When switching to a new cpu_base in switch_hrtimer_base(), we
> briefly enable preemption by unlocking the cpu_base lock in two
> places. During this interval it's possible for the running thread
> to be swapped to a different CPU.
> 
> Consider the following example:
> 
> CPU #0                                 CPU #1
> ----                                   ----
> hrtimer_start()                        ...
>  lock_hrtimer_base()
>  switch_hrtimer_base()
>   this_cpu = 0;
>   target_cpu_base = 0;
>   raw_spin_unlock(&cpu_base->lock)
> <migrate to CPU 1>

Errm. switch_hrtimer_base() is called with interrupts disabled and
they stay disabled, so how exactly is the task going to be migrated?

Thanks,

	tglx
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