testsuite: Fix race condition in check-libthread-db

It is possible for the created thread to reach the breakpoint before
the main thread has set errno to 23.

Prevent this using a pthread barrier.

	* gdb.threads/check-libthread-db.c (thread_routine): Use a
	pthread barrier.
	(main): Likewise.

2 files changed, 14 insertions, 0 deletions

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 2b4b097..2072705 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,9 @@
+2018-09-27  Alan Hayward  <alan.hayward@arm.com>
+
+	* gdb.threads/check-libthread-db.c (thread_routine): Use a
+	pthread barrier.
+	(main): Likewise.
+
 2018-09-26  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* gdb.base/large-frame-1.c: New file.
diff --git a/gdb/testsuite/gdb.threads/check-libthread-db.c b/gdb/testsuite/gdb.threads/check-libthread-db.c
index 85a97a9..2d9aec4 100644
--- a/gdb/testsuite/gdb.threads/check-libthread-db.c
+++ b/gdb/testsuite/gdb.threads/check-libthread-db.c
@@ -23,6 +23,10 @@
 #include <pthread.h>
 #include <errno.h>
 
+/* This barrier ensures we only reach the initial breakpoint after both threads
+   have set errno.  */
+pthread_barrier_t start_threads_barrier;
+
 static void
 break_here (void)
 {
@@ -32,6 +36,7 @@ static void *
 thread_routine (void *arg)
 {
   errno = 42;
+  pthread_barrier_wait (&start_threads_barrier);
 
   break_here ();
 
@@ -47,6 +52,8 @@ main (int argc, char *argv)
   pthread_t the_thread;
   int err;
 
+  pthread_barrier_init (&start_threads_barrier, NULL, 2);
+
   err = pthread_create (&the_thread, NULL, thread_routine, NULL);
   if (err != 0)
     {
@@ -55,6 +62,7 @@ main (int argc, char *argv)
     }
 
   errno = 23;
+  pthread_barrier_wait (&start_threads_barrier);
 
   err = pthread_join (the_thread, NULL);
   if (err != 0)