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

[DDD <dongdong.deng@windriver.com>]

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

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
>