ck_rwlock: Re-observe latch bits if necessary.

include/ck_rwlock.h

@@ -213,8 +213,21 @@ ck_rwlock_read_lock(ck_rwlock_t *rw)
 		 */
 		ck_pr_fence_atomic_load();
 
-		if (ck_pr_load_32(&rw->writer) == 0)
+		if (ck_pr_load_32(&rw->writer) == 0) {
+			/*
+			 * It is possible that a latch operation has itself
+			 * gone unnoticed across a write-side critical section.
+			 * However, the write-side critical section itself
+			 * guarantees an ordering with respect to the previous
+			 * fence operation. If writer is observed to 0, then
+			 * it is guaranteed that the latch will be visible.
+			 */
+			snapshot = ck_pr_load_32(&rw->n_readers);
+			if (snapshot >> CK_RWLOCK_LATCH_SHIFT)
+				continue;
+
 			break;
+		}
 
 		ck_pr_dec_32(&rw->n_readers);
 	}