Re: [PATCH] locking/rwsem: Remove arch specific rwsem files

[Peter Zijlstra]

On Mon, Feb 11, 2019 at 10:36:01AM +0100, Peter Zijlstra wrote:
> On Sun, Feb 10, 2019 at 09:00:50PM -0500, Waiman Long wrote:
> > +static inline int __down_read_trylock(struct rw_semaphore *sem)
> > +{
> > +	long tmp;
> > +
> > +	while ((tmp = atomic_long_read(&sem->count)) >= 0) {
> > +		if (tmp == atomic_long_cmpxchg_acquire(&sem->count, tmp,
> > +				   tmp + RWSEM_ACTIVE_READ_BIAS)) {
> > +			return 1;
> 
> That really wants to be:
> 
> 		if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
> 						    tmp + RWSEM_ACTIVE_READ_BIAS))
> 
> > +		}
> > +	}
> > +	return 0;
> > +}

Also, the is the one case where LL/SC can actually do 'better'. Do you
have benchmarks for say PowerPC or ARM64 ?