[PATCH v2] i386: Use pthread_barrier for synchronization on tst-bz21269

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

To improve the false negative in patchwork buildbot.

Checked on i686-linux-gnu.
---
 sysdeps/unix/sysv/linux/i386/tst-bz21269.c | 62 +++++++---------------
 1 file changed, 19 insertions(+), 43 deletions(-)

diff --git a/sysdeps/unix/sysv/linux/i386/tst-bz21269.c b/sysdeps/unix/sysv/linux/i386/tst-bz21269.c
index 1850240c34..bc2a380a11 100644
--- a/sysdeps/unix/sysv/linux/i386/tst-bz21269.c
+++ b/sysdeps/unix/sysv/linux/i386/tst-bz21269.c
@@ -20,16 +20,13 @@
    more specifically in do_multicpu_tests function.  The main changes
    are:
 
-   - C11 atomics instead of plain access.
+   - Use pthread_barrier instead of atomic and futexes.
    - Remove x86_64 support which simplifies the syscall handling
      and fallbacks.
    - Replicate only the test required to trigger the issue for the
      BZ#21269.  */
 
-#include <stdatomic.h>
-
 #include <asm/ldt.h>
-#include <linux/futex.h>
 
 #include <setjmp.h>
 #include <signal.h>
@@ -40,6 +37,9 @@
 #include <support/xunistd.h>
 #include <support/check.h>
 #include <support/xthread.h>
+#include <support/xsignal.h>
+
+#define NITER 5
 
 static int
 xset_thread_area (struct user_desc *u_info)
@@ -55,13 +55,6 @@ xmodify_ldt (int func, const void *ptr, unsigned long bytecount)
   TEST_VERIFY_EXIT (syscall (SYS_modify_ldt, 1, ptr, bytecount) == 0);
 }
 
-static int
-futex (int *uaddr, int futex_op, int val, void *timeout, int *uaddr2,
-	int val3)
-{
-  return syscall (SYS_futex, uaddr, futex_op, val, timeout, uaddr2, val3);
-}
-
 static void
 xsethandler (int sig, void (*handler)(int, siginfo_t *, void *), int flags)
 {
@@ -69,7 +62,7 @@ xsethandler (int sig, void (*handler)(int, siginfo_t *, void *), int flags)
   sa.sa_sigaction = handler;
   sa.sa_flags = SA_SIGINFO | flags;
   TEST_VERIFY_EXIT (sigemptyset (&sa.sa_mask) == 0);
-  TEST_VERIFY_EXIT (sigaction (sig, &sa, 0) == 0);
+  xsigaction (sig, &sa, 0);
 }
 
 static jmp_buf jmpbuf;
@@ -123,33 +116,24 @@ setup_low_user_desc (void)
   low_user_desc_clear->seg_not_present = 1;
 }
 
-/* Possible values of futex:
-   0: thread is idle.
-   1: thread armed.
-   2: thread should clear LDT entry 0.
-   3: thread should exit.  */
-static atomic_uint ftx;
+static pthread_barrier_t barrier;
 
 static void *
 threadproc (void *ctx)
 {
-  while (1)
+  for (int i = 0; i < NITER; i++)
     {
-      futex ((int *) &ftx, FUTEX_WAIT, 1, NULL, NULL, 0);
-      while (atomic_load (&ftx) != 2)
-	{
-	  if (atomic_load (&ftx) >= 3)
-	    return NULL;
-	}
+      xpthread_barrier_wait (&barrier);
 
       /* clear LDT entry 0.  */
       const struct user_desc desc = { 0 };
       xmodify_ldt (1, &desc, sizeof (desc));
 
-      /* If ftx == 2, set it to zero,  If ftx == 100, quit.  */
-      if (atomic_fetch_add (&ftx, -2) != 2)
-	return NULL;
+      /* Wait for 'ss' set in main thread.  */
+      xpthread_barrier_wait (&barrier);
     }
+
+  return NULL;
 }
 
 
@@ -180,18 +164,19 @@ do_test (void)
   /* Some kernels send SIGBUS instead.  */
   xsethandler (SIGBUS, sigsegv_handler, 0);
 
+  xpthread_barrier_init (&barrier, NULL, 2);
+
   thread = xpthread_create (0, threadproc, 0);
 
   asm volatile ("mov %%ss, %0" : "=rm" (orig_ss));
 
-  for (int i = 0; i < 5; i++)
+  for (int i = 0; i < NITER; i++)
     {
       if (sigsetjmp (jmpbuf, 1) != 0)
 	continue;
 
       /* Make sure the thread is ready after the last test. */
-      while (atomic_load (&ftx) != 0)
-	;
+      xpthread_barrier_wait (&barrier);
 
       struct user_desc desc = {
 	.entry_number       = 0,
@@ -207,28 +192,19 @@ do_test (void)
 
       xmodify_ldt (0x11, &desc, sizeof (desc));
 
-      /* Arm the thread.  */
-      ftx = 1;
-      futex ((int*) &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
+      xpthread_barrier_wait (&barrier);
 
       asm volatile ("mov %0, %%ss" : : "r" (0x7));
 
-      /* Fire up thread modify_ldt call.  */
-      atomic_store (&ftx, 2);
-
-      while (atomic_load (&ftx) != 0)
-	;
-
       /* On success, modify_ldt will segfault us synchronously and we will
 	 escape via siglongjmp.  */
       support_record_failure ();
     }
 
-  atomic_store (&ftx, 100);
-  futex ((int*) &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
-
   xpthread_join (thread);
 
+  xpthread_barrier_destroy (&barrier);
+
   return 0;
 }
 
-- 
2.34.1