Subject: [PATCH] ARM: fix cpu_relax() in case of doing dmb

From: [email-masked] (Hui Wang)
Date: Fri, 24 Aug 2012 09:10:43 +0800

Will Deacon wrote:
> On Wed, Aug 22, 2012 at 03:52:18PM +0100, Shawn Guo wrote:
>   
>> diff --git a/arch/arm/include/asm/processor.h b/arch/arm/include/asm/processor.h
>> index 99afa74..7cc67ce 100644
>> --- a/arch/arm/include/asm/processor.h
>> +++ b/arch/arm/include/asm/processor.h
>> @@ -80,7 +80,14 @@ extern void release_thread(struct task_struct *);
>>  unsigned long get_wchan(struct task_struct *p);
>>  
>>  #if __LINUX_ARM_ARCH__ == 6 || defined(CONFIG_ARM_ERRATA_754327)
>> -#define cpu_relax()			smp_mb()
>> +#define cpu_relax()		do {					\
>> +					asm("nop");			\
>> +					asm("nop");			\
>> +					asm("nop");			\
>> +					asm("nop");			\
>> +					asm("nop");			\
>>     
>
> Can you use nop() instead of the explicit asm? Also, I think we should try
> and use some methodology on deciding the number of nops to insert. Without
> having a full handle on the problem at the moment, it would seem that we
> need at least NR_CPUS worth (since the number of spinning secondaries is
> NR_CPUS-1 and they may execute their barriers in lock-step).
>   
Your concern sounds reasonable, but i did a test, the result show there 
is no explicit relation between NR_CPUS and the number of nop needed.

NR_CPUS = 4 and NR_CPUS = 2 need at least the same number of nop.

Regards,
Hui.

> Will