Implement pthread_rwlock functions for NaCl newlib

src/untrusted/pthread/nc_rwlock.c

Index: src/untrusted/pthread/nc_rwlock.c
diff --git a/src/untrusted/pthread/nc_rwlock.c b/src/untrusted/pthread/nc_rwlock.c
index a824f02a6b3c4e33429d8b903b07be3b0f0094c1..760f2d5fc422c76533eb9a02fd930bb8fa2cb228 100644
--- a/src/untrusted/pthread/nc_rwlock.c
+++ b/src/untrusted/pthread/nc_rwlock.c
@@ -13,9 +13,8 @@
  * in the case of recursive read lock (see read_lock_available)). See:
  * http://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock
  *
- * The thundering herd problem is avoided by only waking a single
- * waiter (either a single writer or a single reader) when the
- * lock is released.
+ * The thundering herd problem is avoided (at least for waiting writers)
+ * by only waking a single writer at a time.
  */
 
 #include <errno.h>
@@ -239,8 +238,11 @@ int pthread_rwlock_unlock(pthread_rwlock_t *rwlock) {
       /* Wake a waiting writer if there is one. */
       rc = pthread_cond_signal(&rwlock->write_possible);
     } else {
-      /* Otherwise wake a waiting reader. */
-      rc = pthread_cond_signal(&rwlock->read_possible);
+      /*
+       * Otherwise wake all waiting readers.  All of them should be able
+       * to make progress now that the write lock is no longer held.
+       */
+      rc = pthread_cond_broadcast(&rwlock->read_possible);
     }
   } else {
     if (rwlock->reader_count == 0) {