Re: [PATCH v2 16/19] ARC: [plat-eznps] Use dedicated cpu_relax()

[Vineet Gupta]

On Monday 09 November 2015 04:15 PM, Peter Zijlstra wrote:
> On Mon, Nov 09, 2015 at 10:22:27AM +0000, Vineet Gupta wrote:
>> On Monday 09 November 2015 03:35 PM, Peter Zijlstra wrote:
>>> On Sat, Nov 07, 2015 at 12:52:34PM +0200, Noam Camus wrote:
>>>> diff --git a/arch/arc/include/asm/processor.h b/arch/arc/include/asm/processor.h
>>>> index 7266ede..50f9bae 100644
>>>> --- a/arch/arc/include/asm/processor.h
>>>> +++ b/arch/arc/include/asm/processor.h
>>>> @@ -58,12 +58,21 @@ struct task_struct;
>>>>   * get optimised away by gcc
>>>>   */
>>>>  #ifdef CONFIG_SMP
>>>> +#ifndef CONFIG_EZNPS_MTM_EXT
>>>>  #define cpu_relax()	__asm__ __volatile__ ("" : : : "memory")
>>>>  #else
>>>> +#define cpu_relax()     \
>>>> +	__asm__ __volatile__ (".word %0" : : "i"(CTOP_INST_SCHD_RW) : "memory")
>>>> +#endif
>>>> +#else
>>>>  #define cpu_relax()	do { } while (0)
>>> I'm fairly sure this is incorrect. Even on UP we expect cpu_relax() to
>>> be a compiler barrier.
>>
>> We discussed this a while back (why do https:/lkml.org/lkml/<year>/.... links work
>> psuedo randomly)
>>
>> http://marc.info/?l=linux-kernel&m=140350765530113
> 
> Hurm.. you have a better memory than me ;-)
> 
> So in general we assume cpu_relax() implies a barrier() and I think we
> have loops like:
> 
> 	while (!var)
> 		cpu_relax();
> 
> where var isn't volatile (or casted using READ_ONCE etc).
> 
> See for instance: kernel/time/timer.c:lock_timer_base() which has:
> 
> 	for (;;) {
> 		u32 tf = timer->flags;
> 
> 		if (!(tf & TIMER_MIGRATING)) {
> 		 ...
> 		}
> 
> 		cpu_relax();
> 	}
> 
> So while TIMER_MIGRATING is set, it will only ever do regular loads,
> which GCC is permitted to lift out if cpu_relax() is not a barrier.

I'll just bite the bullet and make it a compiler barrier and send Linus way in
4.4. Care to provide an Ack or some such.

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