mutex: Fix robust mutex lock acquire (Bug 21778)

65810f0ef0 fixed a robust mutex bug but
introduced BZ 21778: if the CAS used to try to acquire a lock fails, the
expected value is not updated, which breaks other cases in the loce
acquisition loop.  The fix is to simply update the expected value with
the value returned by the CAS, which ensures that behavior is as if the
first case with the CAS never happened (if the CAS fails).

This is a regression introduced in the last release.

Tested on x86_64, i686, ppc64, ppc64le, s390x, aarch64, armv7hl.

ChangeLog

@@ -1,4 +1,17 @@
-2017-07-23  Nathan Rossi  <nathan@nathanrossi.com>
+2017-07-29  Torvald Riegel  <triegel@redhat.com>
+	    Carlos O'Donell  <carlos@redhat.com>
+
+	[BZ 21778]
+	* nptl/pthread_mutex_timedlock.c (__pthread_mutex_timedlock): Update
+	oldval if the CAS fails.
+	* nptl/pthread_mutex_lock.c (__pthread_mutex_lock_full): Likewise.
+	* nptl/tst-mutex7.c: Add comments explaining template test.
+	(ROBUST, DELAY_NSEC, ROUNDS, N): New.
+	(tf, do_test): Use them.
+	* nptl/tst-mutex7robust.c: New file.
+	* nptl/Makefile (tests): Add new test.
+
+2017-07-28  Nathan Rossi  <nathan@nathanrossi.com>
 
 	[BZ #21779]
 	* sysdeps/unix/sysv/linux/microblaze/pt-vfork.S: Branch using PLT.

nptl/Makefile

@@ -230,7 +230,7 @@ LDLIBS-tst-thread_local1 = -lstdc++
 
 tests = tst-attr1 tst-attr2 tst-attr3 tst-default-attr \
 	tst-mutex1 tst-mutex2 tst-mutex3 tst-mutex4 tst-mutex5 tst-mutex6 \
-	tst-mutex7 tst-mutex9 tst-mutex5a tst-mutex7a \
+	tst-mutex7 tst-mutex9 tst-mutex5a tst-mutex7a tst-mutex7robust \
 	tst-mutexpi1 tst-mutexpi2 tst-mutexpi3 tst-mutexpi4 tst-mutexpi5 \
 	tst-mutexpi5a tst-mutexpi6 tst-mutexpi7 tst-mutexpi7a \
 	tst-mutexpi9 \

nptl/pthread_mutex_lock.c

@@ -197,11 +197,14 @@ __pthread_mutex_lock_full (pthread_mutex_t *mutex)
 	{
 	  /* Try to acquire the lock through a CAS from 0 (not acquired) to
 	     our TID | assume_other_futex_waiters.  */
-	  if (__glibc_likely ((oldval == 0)
-			      && (atomic_compare_and_exchange_bool_acq
-				  (&mutex->__data.__lock,
-				   id | assume_other_futex_waiters, 0) == 0)))
-	      break;
+	  if (__glibc_likely (oldval == 0))
+	    {
+	      oldval
+	        = atomic_compare_and_exchange_val_acq (&mutex->__data.__lock,
+	            id | assume_other_futex_waiters, 0);
+	      if (__glibc_likely (oldval == 0))
+		break;
+	    }
 
 	  if ((oldval & FUTEX_OWNER_DIED) != 0)
 	    {

nptl/pthread_mutex_timedlock.c

@@ -154,11 +154,14 @@ __pthread_mutex_timedlock (pthread_mutex_t *mutex,
 	{
 	  /* Try to acquire the lock through a CAS from 0 (not acquired) to
 	     our TID | assume_other_futex_waiters.  */
-	  if (__glibc_likely ((oldval == 0)
-			      && (atomic_compare_and_exchange_bool_acq
-				  (&mutex->__data.__lock,
-				   id | assume_other_futex_waiters, 0) == 0)))
-	      break;
+	  if (__glibc_likely (oldval == 0))
+	    {
+	      oldval
+	        = atomic_compare_and_exchange_val_acq (&mutex->__data.__lock,
+	            id | assume_other_futex_waiters, 0);
+	      if (__glibc_likely (oldval == 0))
+		break;
+	    }
 
 	  if ((oldval & FUTEX_OWNER_DIED) != 0)
 	    {

nptl/tst-mutex7.c

@@ -22,25 +22,41 @@
 #include <stdlib.h>
 #include <time.h>
 
-
+/* This test is a template for other tests to use.  Other tests define
+   the following macros to change the behaviour of the template test.
+   The test is very simple, it configures N threads given the parameters
+   below and then proceeds to go through mutex lock and unlock
+   operations in each thread as described before for the thread
+   function.  */
 #ifndef TYPE
 # define TYPE PTHREAD_MUTEX_DEFAULT
 #endif
-
+#ifndef ROBUST
+# define ROBUST PTHREAD_MUTEX_STALLED
+#endif
+#ifndef DELAY_NSEC
+# define DELAY_NSEC 11000
+#endif
+#ifndef ROUNDS
+# define ROUNDS 1000
+#endif
+#ifndef N
+# define N 100
+#endif
 
 static pthread_mutex_t lock;
 
-
-#define ROUNDS 1000
-#define N 100
-
-
+/* Each thread locks and the subsequently unlocks the lock, yielding
+   the smallest critical section possible.  After the unlock the thread
+   waits DELAY_NSEC nanoseconds before doing the lock and unlock again.
+   Every thread does this ROUNDS times.  The lock and unlock are
+   checked for errors.  */
 static void *
 tf (void *arg)
 {
   int nr = (long int) arg;
   int cnt;
-  struct timespec ts = { .tv_sec = 0, .tv_nsec = 11000 };
+  struct timespec ts = { .tv_sec = 0, .tv_nsec = DELAY_NSEC };
 
   for (cnt = 0; cnt < ROUNDS; ++cnt)
     {
@@ -56,13 +72,16 @@ tf (void *arg)
 	  return (void *) 1l;
 	}
 
-      nanosleep (&ts, NULL);
+      if ((ts.tv_sec > 0) || (ts.tv_nsec > 0))
+	nanosleep (&ts, NULL);
     }
 
   return NULL;
 }
 
-
+/* Setup and run N threads, where each thread does as described
+   in the above thread function.  The threads are given a minimal 1MiB
+   stack since they don't do anything between the lock and unlock.  */
 static int
 do_test (void)
 {
@@ -80,6 +99,12 @@ do_test (void)
       exit (1);
     }
 
+  if (pthread_mutexattr_setrobust (&a, ROBUST) != 0)
+    {
+      puts ("mutexattr_setrobust failed");
+      exit (1);
+    }
+
 #ifdef ENABLE_PI
   if (pthread_mutexattr_setprotocol (&a, PTHREAD_PRIO_INHERIT) != 0)
     {

nptl/tst-mutex7robust.c

@@ -0,0 +1,7 @@
+/* Bug 21778: Fix oversight in robust mutex lock acquisition.  */
+#define TYPE PTHREAD_MUTEX_NORMAL
+#define ROBUST PTHREAD_MUTEX_ROBUST
+#define DELAY_NSEC 0
+#define ROUNDS 1000
+#define N 32
+#include "tst-mutex7.c"