- * We (exclusively) read the old value, and decrement it. If it
- * hits zero, we may have won the lock, so we try (exclusively)
- * storing it.
+ * We exclusively read the old value. If it is zero, we may have
+ * won the lock, so we try exclusively storing it. A memory barrier
+ * is required after we get a lock, and before we release it, because
+ * V6 CPUs are assumed to have weakly ordered memory.