On Thu, Feb 07, 2019 at 08:36:56PM +0100, Peter Zijlstra wrote:

On Thu, Feb 07, 2019 at 02:07:08PM -0500, Waiman Long wrote:

quoted

+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;
	}

N/m that's not in fact the same...

quoted

+	return 0;
+}

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