ARM: why smp_mb() is not needed in the "__mutex_fastpath_lock" and "__mutex_fastpath_unlock" functions

[shan kang]

Yes, a customer reported an issue to me.
After my investigation, I found the problem may be the lack of "dmb"
in mutex_lock and mutex_unlock functions.
Moreover, the issue could be resolved after the "dmb" was added.

2012/7/13 Will Deacon <will.deacon at arm.com>:
> On Fri, Jul 13, 2012 at 10:10:52AM +0100, shan kang wrote:
>> For example, in the following scenario, Process2  may get the wrong value;
>> Process1:
>> mutex_lock(&lock);
>> write data; (store operation)
>> mutex_unlock(&lock);
>>
>> Process2:
>> mutex_lock(&lock);
>> read data; (load operation)
>> mutex_unlock(&lock);
>
> Yes, it looks like we can screw things up in the uncontended case (where
> nobody blocks on the mutex). We could add an smp_mb after the lock operation
> and another one before the unlock, but I'm tempted just to use
> asm-generic/mutex-dec.h instead. The latter approach will subtly change the
> current behaviour, so I'll post a patch when I'm happy with it.
>
> Curious: did you find this by inspection or did you observe it going wrong?
>
> Cheers,
>
> Will