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libitm: Optimize memory order requiremens in gl_wt pre_write.

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

From-SVN: r184400
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Torvald Riegel 2012-02-20 20:56:56 +00:00 • committed by Torvald Riegel
parent 33a03827d2
commit efee0113e8
2 changed files with 18 additions and 3 deletions
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4
libitm/ChangeLog
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@ -1,3 +1,7 @@
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

17
libitm/method-gl.cc
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@ -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.