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From: Michael Ellerman <mpe@ellerman.id.au>
To: Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
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        Viresh Kumar <viresh.kumar@linaro.org>,
        Paul Mackerras <paulus@samba.org>, linuxppc-dev@lists.ozlabs.org
Subject: Re: [patch 05/13] powerpc/smp: Replace open coded task affinity logic
In-Reply-To: <20170412201042.518053336@linutronix.de>
References: <20170412200726.941336635@linutronix.de> <20170412201042.518053336@linutronix.de>
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Date: Thu, 13 Apr 2017 15:47:44 +1000
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Thomas Gleixner <tglx@linutronix.de> writes:

> Init task invokes smp_ops->setup_cpu() from smp_cpus_done(). Init task can
> run on any online CPU at this point, but the setup_cpu() callback requires
> to be invoked on the boot CPU. This is achieved by temporarily setting the
> affinity of the calling user space thread to the requested CPU and reset it
> to the original affinity afterwards.
>
> That's racy vs. CPU hotplug and concurrent affinity settings for that
> thread resulting in code executing on the wrong CPU and overwriting the
> new affinity setting.
>
> That's actually not a problem in this context as neither CPU hotplug nor
> affinity settings can happen, but the access to task_struct::cpus_allowed
> is about to restricted.
>
> Replace it with a call to work_on_cpu_safe() which achieves the same result.
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> Cc: Paul Mackerras <paulus@samba.org>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: linuxppc-dev@lists.ozlabs.org
> ---
>  arch/powerpc/kernel/smp.c |   26 +++++++++++---------------
>  1 file changed, 11 insertions(+), 15 deletions(-)

LGTM.

Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)

cheers