-/* Copyright (C) 2011, 2012 Free Software Foundation, Inc.
+/* Copyright (C) 2011-2018 Free Software Foundation, Inc.
Contributed by Torvald Riegel <triegel@redhat.com>.
This file is part of the GNU Transactional Memory Library (libitm).
static gtm_word clear_locked(gtm_word l) { return l & ~LOCK_BIT; }
// The global ownership record.
- atomic<gtm_word> orec;
+ // No tail-padding necessary (the virtual functions aren't used frequently).
+ atomic<gtm_word> orec __attribute__((aligned(HW_CACHELINE_SIZE)));
virtual void init()
{
virtual void fini() { }
};
-// TODO cacheline padding
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:
- static void pre_write(const void *addr, size_t len)
+ static void pre_write(const void *addr, size_t len,
+ gtm_thread *tx = gtm_thr())
{
- gtm_thread *tx = gtm_thr();
gtm_word v = tx->shared_state.load(memory_order_relaxed);
if (unlikely(!gl_mg::is_locked(v)))
{
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.
tx->undolog.log(addr, len);
}
- static void validate()
+ static void validate(gtm_thread *tx = gtm_thr())
{
// Check that snapshot is consistent. We expect the previous data load to
// have acquire memory order, or be atomic and followed by an acquire
// or read an orec value that was written after the data had been written.
// Either will allow us to detect inconsistent reads because it will have
// a higher/different value.
- gtm_thread *tx = gtm_thr();
gtm_word l = o_gl_mg.orec.load(memory_order_relaxed);
if (l != tx->shared_state.load(memory_order_relaxed))
tx->restart(RESTART_VALIDATE_READ);
static void memtransfer_static(void *dst, const void* src, size_t size,
bool may_overlap, ls_modifier dst_mod, ls_modifier src_mod)
{
- if ((dst_mod != WaW && src_mod != RaW)
- && (dst_mod != NONTXNAL || src_mod == RfW))
- pre_write(dst, size);
+ gtm_thread *tx = gtm_thr();
+ if (dst_mod != WaW && dst_mod != NONTXNAL)
+ pre_write(dst, size, tx);
+ // We need at least undo-logging for an RfW src region because we might
+ // subsequently write there with WaW.
+ if (src_mod == RfW)
+ pre_write(src, size, tx);
// FIXME We should use atomics here (see store()). Let's just hope that
// memcpy/memmove are good enough.
if (src_mod != RfW && src_mod != RaW && src_mod != NONTXNAL
&& dst_mod != WaW)
- validate();
+ validate(tx);
}
static void memset_static(void *dst, int c, size_t size, ls_modifier mod)
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
// See begin_or_restart() for why we need release memory order here.
v = gl_mg::clear_locked(v) + 1;
o_gl_mg.orec.store(v, memory_order_release);
-
- // Need to ensure privatization safety. Every other transaction must
- // have a snapshot time that is at least as high as our commit time
- // (i.e., our commit must be visible to them).
- priv_time = v;
}
+
+ // Need to ensure privatization safety. Every other transaction must have
+ // a snapshot time that is at least as high as our commit time (i.e., our
+ // commit must be visible to them). Because of proxy privatization, we
+ // must ensure that even if we are a read-only transaction. See
+ // ml_wt_dispatch::trycommit() for details: We can't get quite the same
+ // set of problems because we just use one orec and thus, for example,
+ // there cannot be concurrent writers -- but we can still get pending
+ // loads to privatized data when not ensuring privatization safety, which
+ // is problematic if the program unmaps the privatized memory.
+ priv_time = v;
return true;
}
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
// value that is correct wrt. privatization safety.
if (gl_mg::is_locked(v))
{
- // Release the global orec, increasing its version number / timestamp.
- // See begin_or_restart() for why we need release memory order here.
+ // With our rollback, global time increases.
v = gl_mg::clear_locked(v) + 1;
- 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);
-
- // We need a store-load barrier after this store to prevent it
- // from becoming visible after later data loads because the
- // previous value of shared_state has been higher than the actual
- // snapshot time (the lock bit had been set), which could break
- // privatization safety. We do not need a barrier before this
- // store (see pre_write() for an explanation).
- // ??? What is the precise reasoning in the C++11 model?
- atomic_thread_fence(memory_order_seq_cst);
+ // First reset the timestamp published via shared_state. Release
+ // memory order will make this happen after undoing prior data writes.
+ // This must also happen before we actually release the global orec
+ // next, so that future update transactions in other threads observe
+ // a meaningful snapshot time for our transaction; otherwise, they
+ // could read a shared_store value with the LOCK_BIT set, which can
+ // break privatization safety because it's larger than the actual
+ // snapshot time. Note that we only need to consider other update
+ // transactions because only those will potentially privatize data.
+ tx->shared_state.store(v, memory_order_release);
+
+ // Release the global orec, increasing its version number / timestamp.
+ // See begin_or_restart() for why we need release memory order here,
+ // and we also need it to make future update transactions read the
+ // prior update to shared_state too (update transactions acquire the
+ // global orec with acquire memory order).
+ o_gl_mg.orec.store(v, memory_order_release);
}
}
+ virtual bool snapshot_most_recent()
+ {
+ // This is the same check as in validate() except that we do not restart
+ // on failure but simply return the result.
+ return o_gl_mg.orec.load(memory_order_relaxed)
+ == gtm_thr()->shared_state.load(memory_order_relaxed);
+ }
+
+
CREATE_DISPATCH_METHODS(virtual, )
CREATE_DISPATCH_METHODS_MEM()
- gl_wt_dispatch() : abi_dispatch(false, true, false, false, &o_gl_mg)
+ gl_wt_dispatch() : abi_dispatch(false, true, false, false, 0, &o_gl_mg)
{ }
};
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