Re: [PATCH v4 11/16] locking/rwsem: Enable readers spinning on writer

[Peter Zijlstra]

On Sat, Apr 13, 2019 at 01:22:54PM -0400, Waiman Long wrote:
> +/*
> + * Try to acquire read lock before the reader is put on wait queue.
> + * Lock acquisition isn't allowed if the rwsem is locked or a writer handoff
> + * is ongoing.
> + */
> +static inline bool rwsem_try_read_lock_unqueued(struct rw_semaphore *sem)
> +{
> +	long count = atomic_long_read(&sem->count);
> +
> +	if (RWSEM_COUNT_WLOCKED_OR_HANDOFF(count))
> +		return false;
> +
> +	count = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS, &sem->count);
> +	if (!RWSEM_COUNT_WLOCKED_OR_HANDOFF(count)) {
> +		rwsem_set_reader_owned(sem);
> +		lockevent_inc(rwsem_opt_rlock);
> +		return true;
> +	}
> +
> +	/* Back out the change */
> +	atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
> +	return false;
> +}

Doesn't a cmpxchg 'loop' make more sense here?