rwlock: Fix explicit hand-over (bug 21298)

Without this fix, the rwlock can fail to execute the explicit hand-over
in certain cases (e.g., empty critical sections that switch quickly between
read and write phases).  This can then lead to errors in how __wrphase_futex
is accessed, which in turn can lead to deadlocks.

1 files changed, 116 insertions, 0 deletions

diff --git a/nptl/tst-rwlock20.c b/nptl/tst-rwlock20.c
new file mode 100644
index 0000000..4aeea2b
--- /dev/null
+++ b/nptl/tst-rwlock20.c
@@ -0,0 +1,116 @@
+/* Test program for a read-phase / write-phase explicit hand-over.
+   Copyright (C) 2017 Free Software Foundation, Inc.
+
+   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; see the file COPYING.LIB.  If
+   not, see <http://www.gnu.org/licenses/>.  */
+
+#include <errno.h>
+#include <error.h>
+#include <pthread.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <stdint.h>
+#include <time.h>
+#include <atomic.h>
+#include <support/xthread.h>
+
+/* We realy want to set threads to 2 to reproduce this issue. The goal
+   is to have one primary writer and a single reader, and to hit the
+   bug that happens in the interleaving of those two phase transitions.
+   However, on most hardware, adding a second writer seems to help the
+   interleaving happen slightly more often, say 20% of the time.  On a
+   16 core ppc64 machine this fails 100% of the time with an unpatched
+   glibc.  On a 8 core x86_64 machine this fails ~93% of the time, but
+   it doesn't fail at all on a 4 core system, so having available
+   unloaded cores makes a big difference in reproducibility.  On an 8
+   core qemu/kvm guest the reproducer reliability drops to ~10%.  */
+#define THREADS 3
+
+#define KIND PTHREAD_RWLOCK_PREFER_READER_NP
+
+static pthread_rwlock_t lock;
+static int done = 0;
+
+static void*
+tf (void* arg)
+{
+  while (atomic_load_relaxed (&done) == 0)
+    {
+      int rcnt = 0;
+      int wcnt = 100;
+      if ((uintptr_t) arg == 0)
+	{
+	  rcnt = 1;
+	  wcnt = 1;
+	}
+
+      do
+	{
+	  if (wcnt)
+	    {
+	      xpthread_rwlock_wrlock (&lock);
+	      xpthread_rwlock_unlock (&lock);
+	      wcnt--;
+	  }
+	  if (rcnt)
+	    {
+	      xpthread_rwlock_rdlock (&lock);
+	      xpthread_rwlock_unlock (&lock);
+	      rcnt--;
+	  }
+	}
+      while ((atomic_load_relaxed (&done) == 0) && (rcnt + wcnt > 0));
+
+    }
+    return NULL;
+}
+
+
+
+static int
+do_test (void)
+{
+  pthread_t thr[THREADS];
+  int n;
+  pthread_rwlockattr_t attr;
+
+  xpthread_rwlockattr_init (&attr);
+  xpthread_rwlockattr_setkind_np (&attr, KIND);
+
+  xpthread_rwlock_init (&lock, &attr);
+
+  /* Make standard error the same as standard output.  */
+  dup2 (1, 2);
+
+  /* Make sure we see all message, even those on stdout.  */
+  setvbuf (stdout, NULL, _IONBF, 0);
+
+  for (n = 0; n < THREADS; ++n)
+    thr[n] = xpthread_create (NULL, tf, (void *) (uintptr_t) n);
+
+  struct timespec delay;
+  delay.tv_sec = 10;
+  delay.tv_nsec = 0;
+  nanosleep (&delay, NULL);
+  atomic_store_relaxed (&done, 1);
+
+  /* Wait for all the threads.  */
+  for (n = 0; n < THREADS; ++n)
+    xpthread_join (thr[n]);
+
+  return 0;
+}
+
+#include <support/test-driver.c>