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Re: [PATCH] Fix PR target/24071


> The alternative would be, inside the mutex:
>
>    if (__gthread_active < 0) {
>      __gthread_active = 0;
>      __gthrw_(pthread_once) (...);
>    }
>
> and then, instead of "__gthread_active++":
>
>    else
>      __gthread_active = 0;

On second thoughts, I don't think this is safe.  The first test in my patch

  if (__gthread_active < 0)

is not protected to avoid taking a lock on the main code path so  
__gthread_active must never be 0 when the program is threaded.

However I agree that the increment is potentially confusing so we'd probably 
better eliminate it.  I've also decided to make __gthread_active volatile 
for the sake of extra-safety (I think your modification would also have been 
problematic wrt that aspect).  Revised version attached, same testing.

-- 
Eric Botcazou
Index: gthr-posix95.h
===================================================================
--- gthr-posix95.h	(revision 118142)
+++ gthr-posix95.h	(working copy)
@@ -109,6 +109,59 @@ __gthrw(pthread_setschedparam)
 
 #if SUPPORTS_WEAK && GTHREAD_USE_WEAK
 
+/* On Solaris 2.6 up to 9, the libc exposes a POSIX threads interface even if
+   -pthreads is not specified.  The functions are dummies and most return an
+   error value.  However pthread_once returns 0 without invoking the routine
+   it is passed so we cannot pretend that the interface is active if -pthreads
+   is not specified.  On Solaris 2.5.1, the interface is not exposed at all so
+   we need to play the usual game with weak symbols.  On Solaris 10 and up, a
+   working interface is always exposed.  */
+
+#if defined(__sun) && defined(__svr4__)
+
+static volatile int __gthread_active = -1;
+
+static void
+__gthread_trigger (void)
+{
+  __gthread_active = 1;
+}
+
+static inline int
+__gthread_active_p (void)
+{
+  static pthread_mutex_t __gthread_active_mutex = PTHREAD_MUTEX_INITIALIZER;
+  static pthread_once_t __gthread_active_once = PTHREAD_ONCE_INIT;
+
+  /* Avoid reading __gthread_active twice on the main code path.  */
+  int __gthread_active_latest_value = __gthread_active;
+
+  /* This test is not protected to avoid taking a lock on the main code
+     path so every update of __gthread_active in a threaded program must
+     be atomic with regard to the result of the test.  */
+  if (__builtin_expect (__gthread_active_latest_value < 0, 0))
+    {
+      if (__gthrw_(pthread_once))
+	{
+	  /* If this really is a threaded program, then we must ensure that
+	     __gthread_active has been set to 1 before exiting this block.  */
+	  __gthrw_(pthread_mutex_lock) (&__gthread_active_mutex);
+	  __gthrw_(pthread_once) (&__gthread_active_once, __gthread_trigger);
+	  __gthrw_(pthread_mutex_unlock) (&__gthread_active_mutex);
+	}
+
+      /* Make sure we'll never enter this block again.  */
+      if (__gthread_active < 0)
+	__gthread_active = 0;
+
+      __gthread_active_latest_value = __gthread_active;
+    }
+
+  return __gthread_active_latest_value != 0;
+}
+
+#else /* not Solaris */
+
 static inline int
 __gthread_active_p (void)
 {
@@ -117,6 +170,8 @@ __gthread_active_p (void)
   return __gthread_active_ptr != 0;
 }
 
+#endif /* Solaris */
+
 #else /* not SUPPORTS_WEAK */
 
 static inline int
Index: gthr-posix.h
===================================================================
--- gthr-posix.h	(revision 118142)
+++ gthr-posix.h	(working copy)
@@ -142,6 +142,59 @@ __gthrw(pthread_setschedparam)
 
 #if SUPPORTS_WEAK && GTHREAD_USE_WEAK
 
+/* On Solaris 2.6 up to 9, the libc exposes a POSIX threads interface even if
+   -pthreads is not specified.  The functions are dummies and most return an
+   error value.  However pthread_once returns 0 without invoking the routine
+   it is passed so we cannot pretend that the interface is active if -pthreads
+   is not specified.  On Solaris 2.5.1, the interface is not exposed at all so
+   we need to play the usual game with weak symbols.  On Solaris 10 and up, a
+   working interface is always exposed.  */
+
+#if defined(__sun) && defined(__svr4__)
+
+static volatile int __gthread_active = -1;
+
+static void
+__gthread_trigger (void)
+{
+  __gthread_active = 1;
+}
+
+static inline int
+__gthread_active_p (void)
+{
+  static pthread_mutex_t __gthread_active_mutex = PTHREAD_MUTEX_INITIALIZER;
+  static pthread_once_t __gthread_active_once = PTHREAD_ONCE_INIT;
+
+  /* Avoid reading __gthread_active twice on the main code path.  */
+  int __gthread_active_latest_value = __gthread_active;
+
+  /* This test is not protected to avoid taking a lock on the main code
+     path so every update of __gthread_active in a threaded program must
+     be atomic with regard to the result of the test.  */
+  if (__builtin_expect (__gthread_active_latest_value < 0, 0))
+    {
+      if (__gthrw_(pthread_once))
+	{
+	  /* If this really is a threaded program, then we must ensure that
+	     __gthread_active has been set to 1 before exiting this block.  */
+	  __gthrw_(pthread_mutex_lock) (&__gthread_active_mutex);
+	  __gthrw_(pthread_once) (&__gthread_active_once, __gthread_trigger);
+	  __gthrw_(pthread_mutex_unlock) (&__gthread_active_mutex);
+	}
+
+      /* Make sure we'll never enter this block again.  */
+      if (__gthread_active < 0)
+	__gthread_active = 0;
+
+      __gthread_active_latest_value = __gthread_active;
+    }
+
+  return __gthread_active_latest_value != 0;
+}
+
+#else /* not Solaris */
+
 static inline int
 __gthread_active_p (void)
 {
@@ -150,6 +203,8 @@ __gthread_active_p (void)
   return __gthread_active_ptr != 0;
 }
 
+#endif /* Solaris */
+
 #else /* not SUPPORTS_WEAK */
 
 static inline int

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