Re: [PATCH] x86: avoid calling arch_trigger_all_cpu_backtrace() atthe same time on SMP

[DDD]

Ingo Molnar wrote:
> * DDD <dongdong.deng@xxxxxxxxxxxxx> wrote:
>
> > Ingo Molnar wrote:
> > > * Dongdong Deng <dongdong.deng@xxxxxxxxxxxxx> 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.

Yep, the spin_lock of test_and_set_bit() could make sure that.

Thank you very much,
I will send out the new patch quickly. :-)

Dongdong

> Thanks,
>
> 	Ingo

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