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Date:   Thu, 13 Apr 2017 15:47:44 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        "David S. Miller" <davem@...emloft.net>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Len Brown <lenb@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>, Tejun Heo <tj@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [patch 05/13] powerpc/smp: Replace open coded task affinity logic

Thomas Gleixner <tglx@...utronix.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@...utronix.de>
> Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> Cc: Paul Mackerras <paulus@...ba.org>
> Cc: Michael Ellerman <mpe@...erman.id.au>
> Cc: linuxppc-dev@...ts.ozlabs.org
> ---
>  arch/powerpc/kernel/smp.c |   26 +++++++++++---------------
>  1 file changed, 11 insertions(+), 15 deletions(-)

LGTM.

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

cheers

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