libitm: Fixed conversion to C++11 atomics.
libitm/
* beginend.cc (GTM::gtm_thread::begin_transaction): Add comment.
(GTM::gtm_thread::try_commit): Changed memory order.
* config/linux/alpha/futex_bits.h (sys_futex0): Take atomic int
as parameter.
* config/linux/x86/futex_bits.h (sys_futex0): Same.
* config/linux/sh/futex_bits.h (sys_futex0): Same.
* config/linux/futex_bits.h (sys_futex0): Same.
* config/linux/futex.cc (futex_wait, futex_wake): Same.
* config/linux/futex.h (futex_wait, futex_wake): Same.
* config/linux/rwlock.h (gtm_rwlock::writers,
gtm_rwlock::writer_readers, gtm_rwlock::readers): Change to atomic
ints.
* config/linux/rwlock.cc (gtm_rwlock::read_lock,
gtm_rwlock::write_lock_generic, gtm_rwlock::read_unlock,
gtm_rwlock::write_unlock): Fix memory orders and fences.
* config/posix/rwlock.cc (gtm_rwlock::read_lock,
gtm_rwlock::write_lock_generic, gtm_rwlock::read_unlock,
gtm_rwlock::write_unlock): Same.
* config/linux/rwlock.h (gtm_rwlock::summary): Change to atomic int.
* method-gl.cc (gl_mg::init, gl_wt_dispatch::memtransfer_static,
gl_wt_dispatch::memset_static, gl_wt_dispatch::begin_or_restart):
Add comments.
(gl_wt_dispatch::pre_write, gl_wt_dispatch::validate,
gl_wt_dispatch::load, gl_wt_dispatch::store,
gl_wt_dispatch::try_commit, gl_wt_dispatch::rollback): Fix memory
orders and fences. Add comments.
From-SVN: r182674
diff --git a/libitm/beginend.cc b/libitm/beginend.cc
index bcc8516..d0ad5a7 100644
--- a/libitm/beginend.cc
+++ b/libitm/beginend.cc
@@ -259,6 +259,9 @@
else
{
#ifdef HAVE_64BIT_SYNC_BUILTINS
+ // We don't really care which block of TIDs we get but only that we
+ // acquire one atomically; therefore, relaxed memory order is
+ // sufficient.
tx->id = global_tid.fetch_add(tid_block_size, memory_order_relaxed);
tx->local_tid = tx->id + 1;
#else
@@ -471,17 +474,28 @@
// Ensure privatization safety, if necessary.
if (priv_time)
{
+ // There must be a seq_cst fence between the following loads of the
+ // other transactions' shared_state and the dispatch-specific stores
+ // that signal updates by this transaction (e.g., lock
+ // acquisitions). This ensures that if we read prior to other
+ // reader transactions setting their shared_state to 0, then those
+ // readers will observe our updates. We can reuse the seq_cst fence
+ // in serial_lock.read_unlock() however, so we don't need another
+ // one here.
// TODO Don't just spin but also block using cond vars / futexes
// here. Should probably be integrated with the serial lock code.
- // TODO For C++0x atomics, the loads of other threads' shared_state
- // should have acquire semantics (together with releases for the
- // respective updates). But is this unnecessary overhead because
- // weaker barriers are sufficient?
for (gtm_thread *it = gtm_thread::list_of_threads; it != 0;
it = it->next_thread)
{
if (it == this) continue;
- while (it->shared_state.load(memory_order_relaxed) < priv_time)
+ // We need to load other threads' shared_state using acquire
+ // semantics (matching the release semantics of the respective
+ // updates). This is necessary to ensure that the other
+ // threads' memory accesses happen before our actions that
+ // assume privatization safety.
+ // TODO Are there any platform-specific optimizations (e.g.,
+ // merging barriers)?
+ while (it->shared_state.load(memory_order_acquire) < priv_time)
cpu_relax();
}
}