linux-kernel - Re: [tip:sched/urgent] Revert "sched/core: Do not use smp_processor_id() with preempt enabled in smpboot_thread

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Date:   Tue, 4 Oct 2016 10:22:15 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     hpa@...or.com, cchalpha@...il.com, xiaolong.ye@...el.com,
        peterz@...radead.org, linux-kernel@...r.kernel.org,
        kernel@...ivas.org, mingo@...nel.org
cc:     linux-tip-commits@...r.kernel.org
Subject: Re: [tip:sched/urgent] Revert "sched/core: Do not use smp_processor_id()
 with preempt enabled in smpboot_thread_fn()"

On Tue, 4 Oct 2016, tip-bot for Ingo Molnar wrote:
> The original change widens a preempt-off section, to avoid a seemingly unsafe
> smp_processor_id() use.
> 
> During review I overlooked two facts:
> 
>  - The code to calls a non-trivial function callback:
> 
>                                 ht->park(td->cpu);
> 
>    ... which might (and does occasionally) sleep, triggering the warning.
> 
>  - More importantly, as pointed out by Peter Zijlstra, using
>    smp_processor_id() in that context is safe, if it's done from
>    a kernel thread that is pinned to a single CPU - which is the
>    case here.
> 
> So revert to the original code that enables preemption sooner.
> 
> Reported-by: kernel test robot <xiaolong.ye@...el.com>
> Acked-by: Peter Zijlstra <peterz@...radead.org>

Acked-by: Thomas Gleixner <tglx@...utronix.de>