linux-kernel - Re: [BUG] Boot hangs at clocksource_done

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Message-ID: <20150831203250.GH16853@twins.programming.kicks-ass.net>
Date:	Mon, 31 Aug 2015 22:32:50 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alex Thorlton <athorlton@....com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Russ Anderson <rja@....com>,
	Dimitri Sivanich <sivanich@....com>
Subject: Re: [BUG] Boot hangs at clocksource_done_booting on large configs

On Mon, Aug 31, 2015 at 01:04:33PM -0500, Alex Thorlton wrote:
q
> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> index fd643d8..8502521 100644
> --- a/kernel/stop_machine.c
> +++ b/kernel/stop_machine.c
> @@ -417,8 +417,11 @@ static void cpu_stopper_thread(unsigned int cpu)
>  {
>         struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
>         struct cpu_stop_work *work;
> +       unsigned long flags;
>         int ret;
> 
> +       local_irq_save(flags);
> +
>  repeat:
>         work = NULL;
>         spin_lock_irq(&stopper->lock);
> @@ -452,6 +455,8 @@ repeat:
>                 cpu_stop_signal_done(done, true);
>                 goto repeat;
>         }
> +
> +       local_irq_restore(flags);
>  }
> 

So I should probably just go sleep and not say anything.. _but_
*confused*.

That local_irq_save() will disable IRQs over:

	work = NULL;

But that is _all_. The spin_unlock_irq() will re-enable IRQs, after
which things will run as usual.

That local_irq_restore() is a total NOP, IRQs are guaranteed enabled at
the irq_local_save() (otherwise lockdep would've complained about
spin_unlock_irq() unconditionally enabling them) and by the time we get
to the restore that same unlock_irq will have enabled them already.


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