linux-kernel - Re: [PATCHv2] ARM: arch_timer: Silence debug preempt warnings

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Message-ID: <211ac95efd49bd57933aec8c8e746648@localhost>
Date:	Sat, 06 Apr 2013 12:41:21 +0200
From:	Marc Zyngier <marc.zyngier@....com>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mark Rutland <mark.rutland@....com>,
	<linux-arm-msm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCHv2] ARM: arch_timer: Silence debug preempt warnings

On Fri,  5 Apr 2013 13:57:29 -0700, Stephen Boyd <sboyd@...eaurora.org>
wrote:
> Hot-plugging with CONFIG_DEBUG_PREEMPT=y on a device with arm
> architected timers causes a slew of "using smp_processor_id() in
> preemptible" warnings:
> 
>   BUG: using smp_processor_id() in preemptible [00000000] code: sh/111
>   caller is arch_timer_cpu_notify+0x14/0xc8
> 
> This happens because sometimes the cpu notifier,
> arch_timer_cpu_notify(), is called in preemptible context and
> other times in non-preemptible context but we use this_cpu_ptr()
> to retrieve the clockevent in all cases. We're only going to
> actually use the pointer in non-preemptible context though, so
> push the this_cpu_ptr() access down into the cases to force the
> checks to occur only in non-preemptible contexts.
> 
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Marc Zyngier <Marc.Zyngier@....com>
> Signed-off-by: Stephen Boyd <sboyd@...eaurora.org>
> ---
> 
> Changes since v1:
>  * Pushed down this_cpu_ptr and added a comment
> 
>  drivers/clocksource/arm_arch_timer.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/clocksource/arm_arch_timer.c
> b/drivers/clocksource/arm_arch_timer.c
> index d7ad425..a65a710 100644
> --- a/drivers/clocksource/arm_arch_timer.c
> +++ b/drivers/clocksource/arm_arch_timer.c
> @@ -248,14 +248,16 @@ static void __cpuinit arch_timer_stop(struct
> clock_event_device *clk)
>  static int __cpuinit arch_timer_cpu_notify(struct notifier_block *self,
>  					   unsigned long action, void *hcpu)
>  {
> -	struct clock_event_device *evt = this_cpu_ptr(arch_timer_evt);
> -
> +	/*
> +	 * Grab cpu pointer in each case to avoid spurious
> +	 * preemptible warnings
> +	 */
>  	switch (action & ~CPU_TASKS_FROZEN) {
>  	case CPU_STARTING:
> -		arch_timer_setup(evt);
> +		arch_timer_setup(this_cpu_ptr(arch_timer_evt));
>  		break;
>  	case CPU_DYING:
> -		arch_timer_stop(evt);
> +		arch_timer_stop(this_cpu_ptr(arch_timer_evt));
>  		break;
>  	}

Looks good to me.

Acked-by: Marc Zyngier <marc.zyngier@....com>

Cheers,

        M.
-- 
Fast, cheap, reliable. Pick two.
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