linux-kernel - Re: [PATCH] x86: avoid calling arch_trigger_all_cpu

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Date:	Thu, 11 Nov 2010 11:01:09 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	DDD <dongdong.deng@...driver.com>
Cc:	dzickus@...hat.com, peterz@...radead.org, tglx@...utronix.de,
	mingo@...hat.com, hpa@...or.com, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: avoid calling arch_trigger_all_cpu_backtrace() at
 the same time on SMP


* DDD <dongdong.deng@...driver.com> wrote:

> Ingo Molnar wrote:
> >* Dongdong Deng <dongdong.deng@...driver.com> wrote:
> >
> >>+static int backtrace_flag;
> >
> >>+	if (cmpxchg(&backtrace_flag, 0, 1) != 0)
> >
> >Sorry to be a PITA, but i asked for test_and_set() because that's
> >the simplest primitive. cmpxchg() semantics is not nearly as
> >obvious and people regularly get it wrong :-/
> 
> As the 'backtrace_flag' could be accessed by multi-cpus on SMP at
> the same time, I use cmpxchg() for getting a atomic/memory barrier
> operation for 'backtrace_flag' variable.
>
> If we use test_and_set, maybe we need smp_wmb() after test_and_set.
> (If I wrong, please correct me, thanks. :-) )

No, test_and_set_bit() is SMP safe and is an implicit barrier as well - so no 
smp_wmb() or other barriers are needed.

Thanks,

	Ingo
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