[google/gcc-4_7] fix race in __cxa_guard_acquire

[Ollie Wild]

This is a merge of r191125 and r191191 from gcc-4_7-branch.  It fixes
a race in __cxa_guard_acquire which can cause duplicate initialization
of static variables within functions.

Okay for google/gcc-4_7?

Thanks,
Ollie


Google ref b/7173106.

        * libsupc++/guard.cc (__cxa_guard_acquire): Exit the loop earlier if
        we detect that another thread has had success. Don't compare_exchange
        from a finished state back to a waiting state.  Fix up the last
        argument of the first __atomic_compare_exchange_n.  Comment.

commit 40ec687ace62b4d4c64f72be2bdf4321f5213107
Author: Ollie Wild <a...@google.com>
Date:   Wed Sep 19 14:52:53 2012 -0500

    Merge r191125 and r191191 from gcc-4_7-branch.
    
    Google ref b/7173106.
    
        * libsupc++/guard.cc (__cxa_guard_acquire): Exit the loop earlier if
        we detect that another thread has had success. Don't compare_exchange
        from a finished state back to a waiting state.  Fix up the last
        argument of the first __atomic_compare_exchange_n.  Comment.

diff --git a/libstdc++-v3/libsupc++/guard.cc b/libstdc++-v3/libsupc++/guard.cc
index adc9608..f8550c0 100644
--- a/libstdc++-v3/libsupc++/guard.cc
+++ b/libstdc++-v3/libsupc++/guard.cc
@@ -244,16 +244,16 @@ namespace __cxxabiv1
     if (__gthread_active_p ())
       {
        int *gi = (int *) (void *) g;
-       int expected(0);
        const int guard_bit = _GLIBCXX_GUARD_BIT;
        const int pending_bit = _GLIBCXX_GUARD_PENDING_BIT;
        const int waiting_bit = _GLIBCXX_GUARD_WAITING_BIT;
 
        while (1)
          {
+           int expected(0);
            if (__atomic_compare_exchange_n(gi, &expected, pending_bit, false,
                                            __ATOMIC_ACQ_REL,
-                                           __ATOMIC_RELAXED))
+                                           __ATOMIC_ACQUIRE))
              {
                // This thread should do the initialization.
                return 1;
@@ -264,13 +264,26 @@ namespace __cxxabiv1
                // Already initialized.
                return 0;       
              }
+
             if (expected == pending_bit)
               {
+                // Use acquire here.
                 int newv = expected | waiting_bit;
                 if (!__atomic_compare_exchange_n(gi, &expected, newv, false,
                                                  __ATOMIC_ACQ_REL, 
-                                                 __ATOMIC_RELAXED))
-                  continue;
+                                                 __ATOMIC_ACQUIRE))
+                  {
+                    if (expected == guard_bit)
+                      {
+                        // Make a thread that failed to set the
+                        // waiting bit exit the function earlier,
+                        // if it detects that another thread has
+                        // successfully finished initialising.
+                        return 0;
+                      }
+                    if (expected == 0)
+                      continue;
+                  }
                 
                 expected = newv;
               }