libitm: Optimize memory order requiremens in gl_wt pre_write.

	libtim/
	* method-gl.cc (gl_wt_dispatch::pre_write): Optimize memory orders.

From-SVN: r184400

diff --git a/libitm/ChangeLog b/libitm/ChangeLog
index 75410f5..0a06bfe 100644 (file)
--- a/libitm/ChangeLog
+++ b/libitm/ChangeLog
@@ -1,5 +1,9 @@
 2012-02-20  Torvald Riegel  <triegel@redhat.com>
 
+       * method-gl.cc (gl_wt_dispatch::pre_write): Optimize memory orders.
+
+2012-02-20  Torvald Riegel  <triegel@redhat.com>
+
        * method-gl.cc (gl_wt_dispatch::memtransfer_static): Add missing undo
        for RfW src.  Optimize number of calls to gtm_thr.
 

diff --git a/libitm/method-gl.cc b/libitm/method-gl.cc
index 4a18b62..503c19e 100644 (file)
--- a/libitm/method-gl.cc
+++ b/libitm/method-gl.cc
@@ -104,16 +104,27 @@ protected:
          tx->restart(RESTART_VALIDATE_WRITE);
 
        // CAS global orec from our snapshot time to the locked state.
-       // We need acq_rel memory order here to synchronize with other loads
-       // and modifications of orec.
+        // We need acquire memory order here to synchronize with other
+        // (ownership) releases of the orec.  We do not need acq_rel order
+        // because whenever another thread reads from this CAS'
+        // modification, then it will abort anyway and does not rely on
+        // any further happens-before relation to be established.
+       // Also note that unlike in ml_wt's increase of the global time
+       // base (remember that the global orec is used as time base), we do
+       // not need require memory order here because we do not need to make
+       // prior orec acquisitions visible to other threads that try to
+       // extend their snapshot time.
        if (!o_gl_mg.orec.compare_exchange_strong (now, gl_mg::set_locked(now),
-                                                  memory_order_acq_rel))
+                                                  memory_order_acquire))
          tx->restart(RESTART_LOCKED_WRITE);
 
        // We use an explicit fence here to avoid having to use release
        // memory order for all subsequent data stores.  This fence will
        // synchronize with loads of the data with acquire memory order.  See
        // validate() for why this is necessary.
+        // Adding require memory order to the prior CAS is not sufficient,
+        // at least according to the Batty et al. formalization of the
+        // memory model.
        atomic_thread_fence(memory_order_release);
 
        // Set shared_state to new value.