nptl: Fix __futex_clocklock64 return error check [BZ #26964]

The earlier implementation of this, __lll_clocklock, calls lll_clockwait
that doesn't return the futex syscall error codes.  It always tries again
if that fails.

However in the current implementation, when the futex returns EAGAIN,
__futex_clocklock64 will also return EGAIN, even if the futex is taken.

This patch fixes the EAGAIN issue and also adds a check for EINTR.  As
futex syscall can return EINTR if the thread is interrupted by a signal.
In this case I'm assuming the function should continue trying to lock as
there is no mention to about it on POSIX.  Also add a test for both
scenarios.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

3 files changed, 142 insertions, 5 deletions

diff --git a/nptl/Makefile b/nptl/Makefile
index 94d805f0d4..ddd83dfbdd 100644
--- a/nptl/Makefile
+++ b/nptl/Makefile
@@ -298,7 +298,7 @@ tests = tst-attr2 tst-attr3 tst-default-attr \
 	tst-thread-affinity-sched \
 	tst-pthread-defaultattr-free \
 	tst-pthread-attr-sigmask \
-
+	tst-pthread-timedlock-lockloop
 
 tests-container =  tst-pthread-getattr
 
diff --git a/nptl/tst-pthread-timedlock-lockloop.c b/nptl/tst-pthread-timedlock-lockloop.c
new file mode 100644
index 0000000000..bd5ed77a38
--- /dev/null
+++ b/nptl/tst-pthread-timedlock-lockloop.c
@@ -0,0 +1,138 @@
+/* Make sure pthread_mutex_timedlock doesn't return spurious error codes.
+
+   Copyright (C) 2020 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
+   <https://www.gnu.org/licenses/>.  */
+
+#include <errno.h>
+#include <pthread.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <support/check.h>
+#include <support/timespec.h>
+#include <support/xsignal.h>
+#include <support/xthread.h>
+#include <support/xtime.h>
+#include <time.h>
+#include <unistd.h>
+
+#define NANO_PER_SEC 1000000000LL
+#define TIMEOUT (NANO_PER_SEC / 1000LL)
+#define NUM_THREADS 50
+#define RETEST_TIMES 100
+
+static pthread_mutex_t mutex;
+static int runs;
+static clockid_t clockid;
+
+static void
+signal_handler (int sig_num)
+{
+  TEST_COMPARE (sig_num, SIGUSR1);
+}
+
+/* Call pthread_mutex_timedlock()/pthread_mutex_unlock() repetitively, hoping
+   that one of them returns EAGAIN or EINTR unexpectedly.  */
+static void *
+worker_timedlock (void *arg)
+{
+  for (unsigned int run = 0; run < runs; run++)
+    {
+      struct timespec abs_time = timespec_add (xclock_now (CLOCK_REALTIME),
+					       make_timespec (0, 1000000));
+
+      int ret = pthread_mutex_timedlock (&mutex, &abs_time);
+
+      if (ret == 0)
+	xpthread_mutex_unlock (&mutex);
+
+      TEST_VERIFY_EXIT (ret == 0 || ret == ETIMEDOUT);
+    }
+  return NULL;
+}
+
+static void *
+worker_clocklock (void *arg)
+{
+  for (unsigned int run = 0; run < runs; run++)
+    {
+      struct timespec time =
+	timespec_add (xclock_now (clockid), make_timespec (0, 1000000));
+
+      int ret = pthread_mutex_clocklock (&mutex, clockid, &time);
+
+      if (ret == 0)
+	xpthread_mutex_unlock (&mutex);
+
+      TEST_VERIFY_EXIT (ret == 0 || ret == ETIMEDOUT);
+    }
+  return NULL;
+}
+
+static int
+run_test_set (void *(*worker) (void *))
+{
+  pthread_t workers[NUM_THREADS];
+
+  /* Check if default pthread_mutex_{timed,clock}lock with valid arguments
+     returns either 0 or ETIMEDOUT.  Since there is no easy way to force
+     the error condition, the test creates multiple threads which in turn
+     issues pthread_mutex_timedlock multiple times.  */
+  runs = 100;
+  for (int run = 0; run < RETEST_TIMES; run++)
+    {
+      for (int i = 0; i < NUM_THREADS; i++)
+	workers[i] = xpthread_create (NULL, worker, NULL);
+      for (int i = 0; i < NUM_THREADS; i++)
+	xpthread_join (workers[i]);
+    }
+
+  /* The idea is similar to previous tests, but we check if
+     pthread_mutex_{timed,clock}lock does not return EINTR.  */
+  pthread_t thread;
+  runs = 1;
+  for (int i = 0; i < RETEST_TIMES * 1000; i++)
+    {
+      xpthread_mutex_lock (&mutex);
+      thread = xpthread_create (NULL, worker, NULL);
+      /* Sleep just a little bit to reach the lock on the worker thread.  */
+      usleep (10);
+      pthread_kill (thread, SIGUSR1);
+      xpthread_mutex_unlock (&mutex);
+      xpthread_join (thread);
+    }
+
+  return 0;
+}
+
+static int
+do_test (void)
+{
+
+  xsignal (SIGUSR1, signal_handler);
+
+  xpthread_mutex_init (&mutex, NULL);
+
+  run_test_set (worker_timedlock);
+  clockid = CLOCK_REALTIME;
+  run_test_set (worker_clocklock);
+  clockid = CLOCK_MONOTONIC;
+  run_test_set (worker_clocklock);
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/sysdeps/nptl/futex-internal.h b/sysdeps/nptl/futex-internal.h
index 1640da0ce8..9974477c48 100644
--- a/sysdeps/nptl/futex-internal.h
+++ b/sysdeps/nptl/futex-internal.h
@@ -420,19 +420,18 @@ static __always_inline int
 __futex_clocklock64 (int *futex, clockid_t clockid,
                      const struct __timespec64 *abstime, int private)
 {
-  int err = 0;
-
   if (__glibc_unlikely (atomic_compare_and_exchange_bool_acq (futex, 1, 0)))
     {
       while (atomic_exchange_acq (futex, 2) != 0)
         {
+	  int err = 0;
           err = __futex_abstimed_wait64 ((unsigned int *) futex, 2, clockid,
 					 abstime, private);
           if (err == EINVAL || err == ETIMEDOUT || err == EOVERFLOW)
-            break;
+            return err;
         }
     }
-  return err;
+  return 0;
 }
 
 #endif  /* futex-internal.h */