Implement pthread_rwlock functions for NaCl newlib

src/untrusted/pthread/nc_rwlock.c

 /*
  * Copyright 2015 The Native Client Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 /*
  * Native Client rwlock implementation
  *
  * This implementation is a 'write-preferring' reader-writer lock which
  * avoids writer starvation by preventing readers from acquiring the lock
  * while there are waiting writers (with an exception to prevent deadlocks
  * 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>
 
 #include "native_client/src/untrusted/pthread/pthread.h"
 #include "native_client/src/untrusted/pthread/pthread_internal.h"
 #include "native_client/src/untrusted/pthread/pthread_types.h"
 
 int pthread_rwlockattr_init(pthread_rwlockattr_t *attr) {
   attr->type = PTHREAD_PROCESS_PRIVATE;
@@ skipping 203 matching lines @@
       rc = EPERM;
       goto done;
     }
 
     /* Release write lock. */
     rwlock->writer_thread_id = NACL_PTHREAD_ILLEGAL_THREAD_ID;
     if (rwlock->writers_waiting > 0) {
       /* 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) {
       rc = EPERM;
       goto done;
     }
 
     /* Release read lock. */
     rwlock->reader_count--;
     __nc_get_tdb()->rdlock_count--;
@@ skipping 30 matching lines @@
 
   /* Finally unlock the mutex and destroy it. */
   pthread_mutex_unlock(&rwlock->mutex);
   int rc3 = pthread_mutex_destroy(&rwlock->mutex);
   if (rc1 != 0)
     return rc1;
   if (rc2 != 0)
     return rc2;
   return rc3;
 }