Subject: Re: [PATCH-tip 04/22] locking/rwsem: Remove arch specific rwsem files

From: Peter Zijlstra <[email-masked]>
Date: Thu, 7 Feb 2019 20:36:56 +0100

On Thu, Feb 07, 2019 at 02:07:08PM -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;
> +		}
> +	}

Nah, you're supposed to write that like:

	for (;;) {
		val = atomic_long_cond_read_relaxed(&sem->count, VAL < 0);
		if (atomic_long_try_cmpxchg_acquire(&sem->count, &val,
						    val + RWSEM_ACTIVE_READ_BIAS))
			break;
	}

> +	return 0;
> +}

Anyway, yuck, you're keeping all that BIAS nonsense :/ I was so hoping
for a rwsem implementation without that impenetrable crap.