[patch] powerpc: implement optimised mutex fastpaths

[Nick Piggin]

On Thu, Nov 06, 2008 at 03:09:08PM +1100, Paul Mackerras wrote:
> Nick Piggin writes:
> 
> > On Sun, Oct 12, 2008 at 07:47:32AM +0200, Nick Piggin wrote:
> > > 
> > > Implement a more optimal mutex fastpath for powerpc, making use of acquire
> > > and release barrier semantics. This takes the mutex lock+unlock benchmark
> > > from 203 to 173 cycles on a G5.
> > > 
> > > +static inline int
> > > +__mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
> > > +{
> > > +	if (likely(__mutex_cmpxchg_lock(count, 1, 0) == 1))
> > > +		return 1;
> > 
> > Oops, I must have sent the wrong version. This needs a return 0 here.
> 
> Are you going to send the right version?  (or did you send it and I
> missed it?)

Hmm, I thought I did but perhaps not. Will do...


> Also I note that the comment you added just before that routine says:
> 
> + * Change the count from 1 to a value lower than 1, and return 0 (failure)
> + * if it wasn't 1 originally, or return 1 (success) otherwise. This function
> + * MUST leave the value lower than 1 even when the "1" assertion wasn't true.
> 
> yet just doing a __mutex_cmpxchg_lock isn't going to do that.  So
> please fix either the comment or the code (or both :).

OK.