Fix lost wake-up when pthread_rwlock_timedrwlock times out.

If we set up a rwlock to prefer writers (and disallow recursive rdlock
acquisitions), then readers will block for writers that are blocked to
acquire the lock (otherwise, readers could constantly enter and exit,
and the writer would never get the lock).  However, the existing
implementation did not wake such readers when the writer timed out.
This patch adds the missing wake-up.
There's no similar case for writers being blocked on readers.

1 files changed, 116 insertions, 0 deletions

diff --git a/nptl/tst-rwlock15.c b/nptl/tst-rwlock15.c
new file mode 100644
index 0000000000..b292701e21
--- /dev/null
+++ b/nptl/tst-rwlock15.c
@@ -0,0 +1,116 @@
+/* Copyright (C) 2015 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+/* This tests that a writer that is preferred -- but times out due to a
+   reader being present -- does not miss to wake other readers blocked on the
+   writer's pending lock acquisition.  */
+
+#include <errno.h>
+#include <pthread.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+
+/* The bug existed in the code that strictly prefers writers over readers.  */
+static pthread_rwlock_t r = PTHREAD_RWLOCK_WRITER_NONRECURSIVE_INITIALIZER_NP;
+
+static void *
+writer (void *arg)
+{
+  struct timespec ts;
+  if (clock_gettime (CLOCK_REALTIME, &ts) != 0)
+    {
+      puts ("clock_gettime failed");
+      exit (EXIT_FAILURE);
+    }
+  ts.tv_sec += 1;
+  int e = pthread_rwlock_timedwrlock (&r, &ts);
+  if (e != ETIMEDOUT)
+    {
+      puts ("timedwrlock did not time out");
+      exit (EXIT_FAILURE);
+    }
+  return NULL;
+}
+
+static void *
+reader (void *arg)
+{
+  /* This isn't a reliable way to get the interleaving we need (because a
+     failed trylock doesn't synchronize with the writer, and because we could
+     try to lock after the writer has already timed out).  However, both will
+     just lead to false positives.  */
+  int e;
+  while ((e = pthread_rwlock_tryrdlock (&r)) != EBUSY)
+    {
+      if (e != 0)
+	exit (EXIT_FAILURE);
+      pthread_rwlock_unlock (&r);
+    }
+  e = pthread_rwlock_rdlock (&r);
+  if (e != 0)
+    {
+      puts ("reader rdlock failed");
+      exit (EXIT_FAILURE);
+    }
+  pthread_rwlock_unlock (&r);
+  return NULL;
+}
+
+
+static int
+do_test (void)
+{
+  /* Grab a rdlock, then create a writer and a reader, and wait until they
+     finished.  */
+
+  if (pthread_rwlock_rdlock (&r) != 0)
+    {
+      puts ("initial rdlock failed");
+      return 1;
+    }
+
+  pthread_t thw;
+  if (pthread_create (&thw, NULL, writer, NULL) != 0)
+    {
+      puts ("create failed");
+      return 1;
+    }
+  pthread_t thr;
+  if (pthread_create (&thr, NULL, reader, NULL) != 0)
+    {
+      puts ("create failed");
+      return 1;
+    }
+
+  if (pthread_join (thw, NULL) != 0)
+    {
+      puts ("writer join failed");
+      return 1;
+    }
+  if (pthread_join (thr, NULL) != 0)
+    {
+      puts ("reader join failed");
+      return 1;
+    }
+
+  return 0;
+}
+
+
+#define TEST_FUNCTION do_test ()
+#include "../test-skeleton.c"