i386: Use pthread_barrier for synchronization on tst-bz21269

So I was able to reproduce the hangs in the original source, and debug
it, and fix it.  In doing so, I realized that we can't use anything
complex to trigger the thread because that "anything" might also cause
the expected segfault and force everything out of sync again.

Here's what I ended up with, and it doesn't seem to hang where the
original one hung quite often (in a tight while..end loop).  The key
changes are:

1. Calls to futex are error checked, with retries, to ensure that the
   futexes are actually doing what they're supposed to be doing.  In the
   original code, nearly every futex call returned an error.

2. The main loop has checks for whether the thread ran or not, and
   "unlocks" the thread if it didn't (this is how the original source
   hangs).

Note: the usleep() is not for timing purposes, but just to give the
kernel an excuse to run the other thread at that time.  The test will
not hang without it, but is more likely to test the right bugfix
if the usleep() is present.

1 files changed, 28 insertions, 5 deletions

diff --git a/sysdeps/unix/sysv/linux/i386/tst-bz21269.c b/sysdeps/unix/sysv/linux/i386/tst-bz21269.c
index 1850240c34..51d4a1b082 100644
--- a/sysdeps/unix/sysv/linux/i386/tst-bz21269.c
+++ b/sysdeps/unix/sysv/linux/i386/tst-bz21269.c
@@ -135,7 +135,14 @@ threadproc (void *ctx)
 {
   while (1)
     {
-      futex ((int *) &ftx, FUTEX_WAIT, 1, NULL, NULL, 0);
+      /* Continue to wait here until we've successfully waited, unless
+	 we're supposed to be clearing the LDT already.  */
+      while (futex ((int *) &ftx, FUTEX_WAIT, 1, NULL, NULL, 0) < 0)
+	if (atomic_load (&ftx) >= 2)
+	  break;
+
+      /* Normally there's time to hit this busy loop and wait for ftx
+	 to be set to 2.  */
       while (atomic_load (&ftx) != 2)
 	{
 	  if (atomic_load (&ftx) >= 3)
@@ -189,7 +196,14 @@ do_test (void)
       if (sigsetjmp (jmpbuf, 1) != 0)
 	continue;
 
-      /* Make sure the thread is ready after the last test. */
+      /* We may have longjmp'd before triggering the thread.  If so,
+	 trigger the thread now and wait for it.  */
+      if (atomic_load (&ftx) == 1)
+	atomic_store (&ftx, 2);
+
+      /* Make sure the thread is ready after the last test.  FTX is
+	 initially zero for the first loop, and set to zero each time
+	 the thread clears the LDT.  */
       while (atomic_load (&ftx) != 0)
 	;
 
@@ -207,15 +221,24 @@ do_test (void)
 
       xmodify_ldt (0x11, &desc, sizeof (desc));
 
-      /* Arm the thread.  */
-      ftx = 1;
-      futex ((int*) &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
+      /* Arm the thread.  We loop here until we've woken up one thread.  */
+      atomic_store (&ftx, 1);
+      while (futex ((int*) &ftx, FUTEX_WAKE, 1, NULL, NULL, 0) < 1)
+	;
+
+      /* Give the thread a chance to get into it's busy loop.  */
+      usleep (5);
 
+      /* At *ANY* point after this instruction, we may segfault and
+	 longjump back to the top of the loop.  The intention is to
+	 have this happen when the thread clears the LDT, but it could
+	 happen elsewhen.  */
       asm volatile ("mov %0, %%ss" : : "r" (0x7));
 
       /* Fire up thread modify_ldt call.  */
       atomic_store (&ftx, 2);
 
+      /* And wait for it.  */
       while (atomic_load (&ftx) != 0)
 	;