From: Thiago Macieira <thiago.macieira@intel.com>
Date: Thu, 6 Sep 2012 20:31:08 +0000 (+0000)
Subject: re PR libstdc++/54172 (__cxa_guard_acquire thread-safety issue)
X-Git-Tag: misc/gccgo-go1_1_2~1005
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=fa129f20b3d956e6e8a3a017c57e0c62a8ba3002;p=thirdparty%2Fgcc.git

re PR libstdc++/54172 (__cxa_guard_acquire thread-safety issue)

2012-09-06  Thiago Macieira  <thiago.macieira@intel.com>

	PR libstdc++/54172
        * 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. Comment.

From-SVN: r191042
---

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index 707e474b8995..f3a5852dd218 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,10 @@
+2012-09-06  Thiago Macieira  <thiago.macieira@intel.com>
+
+	PR libstdc++/54172
+        * 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. Comment.
+
 2012-09-05  François Dumont  <fdumont@gcc.gnu.org>
 
 	PR libstdc++/54296
diff --git a/libstdc++-v3/libsupc++/guard.cc b/libstdc++-v3/libsupc++/guard.cc
index adc960861081..60165cdecb45 100644
--- a/libstdc++-v3/libsupc++/guard.cc
+++ b/libstdc++-v3/libsupc++/guard.cc
@@ -244,13 +244,13 @@ 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))
@@ -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;
 	       }