linux-kernel - RE: [PATCH v2] oprofile: add SMP barriers for hrtimer hotplug code

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Date:	Tue, 1 Feb 2011 10:57:32 +0530
From:	Santosh Shilimkar <santosh.shilimkar@...com>
To:	Will Deacon <will.deacon@....com>, linux-kernel@...r.kernel.org,
	oprofile-list@...ts.sf.net
Cc:	Robert Richter <robert.richter@....com>
Subject: RE: [PATCH v2] oprofile: add SMP barriers for hrtimer hotplug code

> -----Original Message-----
> From: Will Deacon [mailto:will.deacon@....com]
> Sent: Monday, January 31, 2011 11:23 PM
> To: linux-kernel@...r.kernel.org; oprofile-list@...ts.sf.net
> Cc: Will Deacon; Santosh Shilimkar; Robert Richter
> Subject: [PATCH v2] oprofile: add SMP barriers for hrtimer hotplug
> code
>
> OProfile uses a CPU notifier to start and stop any hrtimers when
> CPUs change
> between ONLINE and DEAD. A static int ctr_running is used to keep
> track of
> the counter state.
>
> This can lead to problems where writes to the state variable are re-
> ordered
> with repect to reads of the variable occurring on other CPUs,
> meaning that
> __oprofile_hrtimer_start may read ctr_running as 0 and not
> initialise the
> hrtimer. Potential deadlock can occur in __oprofile_hrtimer_stop
> because
> lock_hrtimer_base will poll until timer->base != NULL, which will
> never
> happen.
>
> This patch adds an smp_mb() before initialising the hrtimers to
> ensure that
> ctr_running mirrors the correct counter state.
>
> Cc: Santosh Shilimkar <santosh.shilimkar@...com>
> Cc: Robert Richter <robert.richter@....com>
> Signed-off-by: Will Deacon <will.deacon@....com>
> ---
>
Acked-by: Santosh Shilimkar <santosh.shilimkar@...com>

> Changes since v1: Removed redundant barrier from stop() code.
>
>  drivers/oprofile/timer_int.c |    1 +
>  1 files changed, 1 insertions(+), 0 deletions(-)
>
> diff --git a/drivers/oprofile/timer_int.c
> b/drivers/oprofile/timer_int.c
> index 0107251..a3a94ea 100644
> --- a/drivers/oprofile/timer_int.c
> +++ b/drivers/oprofile/timer_int.c
> @@ -48,6 +48,7 @@ static int oprofile_hrtimer_start(void)
>  {
>  	get_online_cpus();
>  	ctr_running = 1;
> +	smp_mb();
>  	on_each_cpu(__oprofile_hrtimer_start, NULL, 1);
>  	put_online_cpus();
>  	return 0;
> --
> 1.7.0.4
>
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