[PATCH] gdb/testsuite: fix race condition in gdb.replay/missing-thread.c

[simon.marchi@polymtl.ca]

From: Simon Marchi <simon.marchi@polymtl.ca>

I get frequent failures in gdb.replay/missing-thread.exp, starting with:

    FAIL: gdb.replay/missing-thread.exp: non_stop=on: record_initial_logfile: continue (timeout)

I tracked this down to a race condition in the test program, causing it
not to generate the expected SIGTRAP.  This can be reproduced by adding
a sleep(1) just after the pthread_create.

The expected behavior is:

 - The main thread starts a second thread
 - The main thread blocks on pthread_cond_wait, waiting to be notified
   by the second thread (to make sure the second thread had time to
   start)
 - The second thread notifies the main thread using pthread_cond_signal
 - The main thread sends a SIGTRAP to the second thread

However, this can happen:

 - The main thread starts a second thread
 - The second thread calls pthread_cond_signal, while no one is waiting
   to the condvar
 - The main thread blocks on pthread_cond_wait forever

Fix it by introducing a separate boolean predicate protected by the
shared mutex, and looping until it is true.  This is the way
pthread_cond_wait is meant to be used. From pthread_cond_wait(3):

    When using condition variables there is always a boolean predicate
    involving shared variables associated with each condition wait that
    is true if the thread should proceed. Spurious wakeups from the
    pthread_cond_wait() or pthread_cond_timedwait() functions may occur.
    Since the return from pthread_cond_wait() or
    pthread_cond_timedwait() does not imply anything about the value of
    this predicate, the predicate should be re-evaluated upon such
    return.

Finally, make things static, just out of principle, and fix minor
formatting issues.

Change-Id: I62ba2085a2d506dc3d91e32cd5de48c43a3ff55e
---
 gdb/testsuite/gdb.replay/missing-thread.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/gdb/testsuite/gdb.replay/missing-thread.c b/gdb/testsuite/gdb.replay/missing-thread.c
index 0a17be1c7dd1..aad6396f24c2 100644
--- a/gdb/testsuite/gdb.replay/missing-thread.c
+++ b/gdb/testsuite/gdb.replay/missing-thread.c
@@ -22,23 +22,28 @@
 #include <unistd.h>
 #include <stdbool.h>
 
-pthread_mutex_t g_mutex = PTHREAD_MUTEX_INITIALIZER;
-pthread_cond_t g_condvar = PTHREAD_COND_INITIALIZER;
+static pthread_mutex_t g_mutex = PTHREAD_MUTEX_INITIALIZER;
+static pthread_cond_t g_condvar = PTHREAD_COND_INITIALIZER;
+static bool g_ready = false;
 
-void *
+static void *
 worker_function (void *arg)
 {
   printf ("In worker, about to notify\n");
+
+  pthread_mutex_lock (&g_mutex);
+  g_ready = true;
+  pthread_mutex_unlock (&g_mutex);
   pthread_cond_signal (&g_condvar);
 
   while (true)
-    sleep(1);
+    sleep (1);
 
   return NULL;
 }
 
 int
-main()
+main ()
 {
   pthread_t my_thread;
 
@@ -46,7 +51,8 @@ main()
   assert (result == 0);
 
   pthread_mutex_lock (&g_mutex);
-  pthread_cond_wait (&g_condvar, &g_mutex);
+  while (!g_ready)
+    pthread_cond_wait (&g_condvar, &g_mutex);
 
   printf ("In main, have been woken.\n");
   pthread_mutex_unlock (&g_mutex);

base-commit: deb47060b5812c6c41ecc790cb28898fcbc45c93
-- 
2.53.0