[PATCH] powerpc: Use lwarx hint bit in spinlocks

[Josh Boyer]

On Wed, Feb 10, 2010 at 01:50:11PM +1100, Anton Blanchard wrote:
>
>Recent versions of the PowerPC architecture added a hint bit to the larx
>instructions to differentiate between an atomic operation and a lock operation:
>
>> 0 Other programs might attempt to modify the word in storage addressed by EA
>> even if the subsequent Store Conditional succeeds.
>> 
>> 1 Other programs will not attempt to modify the word in storage addressed by
>> EA until the program that has acquired the lock performs a subsequent store
>> releasing the lock.
>
>To avoid a binutils dependency this patch create macros for the extended lwarx
>format and uses it in the spinlock code. To test this change I used a simple
>test case that acquires and releases a global pthread mutex:
>
>	pthread_mutex_lock(&mutex);
>	pthread_mutex_unlock(&mutex);
>
>On a 32 core POWER6 running 32 test threads we spend almost all our time in
>the futex spinlock code (as expected):
>
>    94.37%     perf  [kernel]                     [k] ._raw_spin_lock
>               |          
>               |--99.95%-- ._raw_spin_lock
>               |          |          
>               |          |--63.29%-- .futex_wake
>               |          |          
>               |          |--36.64%-- .futex_wait_setup
>
>which is a good test for this patch. The results (in lock/unlock operations
>per second) are:
>
>before: 1538203 ops/sec
>after:  2189219 ops/sec
>
>An improvement of 42%
>
>Signed-off-by: Anton Blanchard <anton at samba.org>
>---
>
>We've had issues in the past with chips that do the wrong thing and don't
>ignore instruction reserved bits. One option might be to limit this change to
>64bit and verify on that limited set of CPUs.

In the off chance that someone actually does an SMP 44x, I think the hint bit
here would just be ignored (I could test possibly if we want to verify).
However, I thought the FSL parts didn't like toggling the reserved bits and
those already are SMP in some cases.

Kumar?

josh