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

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Message-ID: <1296580107.13713.1.camel@e102144-lin.cambridge.arm.com>
Date:	Tue, 01 Feb 2011 17:08:27 +0000
From:	Will Deacon <will.deacon@....com>
To:	Ming Lei <tom.leiming@...il.com>
Cc:	Santosh Shilimkar <santosh.shilimkar@...com>,
	linux-kernel@...r.kernel.org, oprofile-list@...ts.sf.net,
	Robert Richter <robert.richter@....com>
Subject: Re: [PATCH v2] oprofile: add SMP barriers for hrtimer hotplug code

On Tue, 2011-02-01 at 08:03 +0000, Ming Lei wrote:
> Hi,
> 
Hello Lei Ming,

> 2011/2/1 Santosh Shilimkar <santosh.shilimkar@...com>:
> >> -----Original Message-----
> >> From: Will Deacon [mailto:will.deacon@....com]
> >> 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);
> 
> Just a little question:
> 
> Before calling __oprofile_hrtimer_start in other CPUs,
> there have been some smp_mb executed already, such as called by
> smp_call_function_many in current CPU, and called by
> generic_smp_call_function_interrupt in the func-calling CPU,
> so are these smp_mb enough for correct order of access
> for 'ctr_running' ?

I didn't dive into the guts of the cross-calling implementation, but it
appears that you're correct. There's also the csd_{un}lock functions
which have smp_mb() in them too, so I think we have plenty of barrier
goodness as it is.

I'll trash the patch.

Cheers,

Will


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