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libitm: Remove obsolete handling of prior serial lock corner cases in gl_wt.

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libitm/
	* method-gl.cc (gl_wt_dispatch::trycommit): Remove handling of
	serial mode corner cases made obsolete by prior gtm_rwlock changes.
	(gl_wt_dispatch.rollback): Same.

From-SVN: r184401
This commit is contained in:
Torvald Riegel 2012-02-20 20:57:09 +00:00 • committed by Torvald Riegel
parent efee0113e8
commit 651ff4152e
2 changed files with 11 additions and 28 deletions
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6
libitm/ChangeLog
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@ -1,3 +1,9 @@
2012-02-20 Torvald Riegel <triegel@redhat.com>
* method-gl.cc (gl_wt_dispatch::trycommit): Remove handling of
serial mode corner cases made obsolete by prior gtm_rwlock changes.
(gl_wt_dispatch.rollback): Same.
2012-02-20 Torvald Riegel <triegel@redhat.com>
* method-gl.cc (gl_wt_dispatch::pre_write): Optimize memory orders.

33
libitm/method-gl.cc
View file

@ -76,12 +76,10 @@ static gl_mg o_gl_mg;
// validate that no other update transaction comitted before we acquired the
// orec, so we have the most recent timestamp and no other transaction can
// commit until we have committed).
// However, we therefore cannot use this method for a serial transaction
// (because shared_state needs to remain at ~0) and we have to be careful
// when switching to serial mode (see the special handling in trycommit() and
// rollback()).
// ??? This sharing adds some complexity wrt. serial mode. Just use a separate
// state variable?
// However, we therefore depend on shared_state not being modified by the
// serial lock during upgrades to serial mode, which is ensured by
// gtm_thread::serialirr_mode by not calling gtm_rwlock::write_upgrade_finish
// before we have committed or rolled back.
class gl_wt_dispatch : public abi_dispatch
{
protected:
@ -283,15 +281,6 @@ public:
gtm_thread* tx = gtm_thr();
gtm_word v = tx->shared_state.load(memory_order_relaxed);
// Special case: If shared_state is ~0, then we have acquired the
// serial lock (tx->state is not updated yet). In this case, the previous
// value isn't available anymore, so grab it from the global lock, which
// must have a meaningful value because no other transactions are active
// anymore. In particular, if it is locked, then we are an update
// transaction, which is all we care about for commit.
if (v == ~(typeof v)0)
v = o_gl_mg.orec.load(memory_order_relaxed);
// Release the orec but do not reset shared_state, which will be modified
// by the serial lock right after our commit anyway. Also, resetting
// shared state here would interfere with the serial lock's use of this
@ -319,15 +308,6 @@ public:
gtm_thread *tx = gtm_thr();
gtm_word v = tx->shared_state.load(memory_order_relaxed);
// Special case: If shared_state is ~0, then we have acquired the
// serial lock (tx->state is not updated yet). In this case, the previous
// value isn't available anymore, so grab it from the global lock, which
// must have a meaningful value because no other transactions are active
// anymore. In particular, if it is locked, then we are an update
// transaction, which is all we care about for rollback.
bool is_serial = v == ~(typeof v)0;
if (is_serial)
v = o_gl_mg.orec.load(memory_order_relaxed);
// Release lock and increment version number to prevent dirty reads.
// Also reset shared state here, so that begin_or_restart() can expect a
@ -340,10 +320,7 @@ public:
o_gl_mg.orec.store(v, memory_order_release);
// Also reset the timestamp published via shared_state.
// Special case: Only do this if we are not a serial transaction
// because otherwise, we would interfere with the serial lock.
if (!is_serial)
tx->shared_state.store(v, memory_order_release);
tx->shared_state.store(v, memory_order_release);
// We need a store-load barrier after this store to prevent it
// from becoming visible after later data loads because the