blob: 4b6769ba2c00a3ff9db724595b5a9d96e376ec92 [file] [log] [blame]
/* Copyright (C) 2011, 2012 Free Software Foundation, Inc.
Contributed by Torvald Riegel <>.
This file is part of the GNU Transactional Memory Library (libitm).
Libitm is free software; you can redistribute it and/or modify it
under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 of the License, or
(at your option) any later version.
Libitm is distributed in the hope that it will be useful, but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
FOR A PARTICULAR PURPOSE. See the GNU General Public License for
more details.
Under Section 7 of GPL version 3, you are granted additional
permissions described in the GCC Runtime Library Exception, version
3.1, as published by the Free Software Foundation.
You should have received a copy of the GNU General Public License and
a copy of the GCC Runtime Library Exception along with this program;
see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
<>. */
#include "libitm_i.h"
using namespace GTM;
namespace {
// This group consists of all TM methods that synchronize via just a single
// global lock (or ownership record).
struct gl_mg : public method_group
static const gtm_word LOCK_BIT = (~(gtm_word)0 >> 1) + 1;
// We can't use the full bitrange because ~0 in gtm_thread::shared_state has
// special meaning.
static const gtm_word VERSION_MAX = (~(gtm_word)0 >> 1) - 1;
static bool is_locked(gtm_word l) { return l & LOCK_BIT; }
static gtm_word set_locked(gtm_word l) { return l | LOCK_BIT; }
static gtm_word clear_locked(gtm_word l) { return l & ~LOCK_BIT; }
// The global ownership record.
// No tail-padding necessary (the virtual functions aren't used frequently).
atomic<gtm_word> orec __attribute__((aligned(HW_CACHELINE_SIZE)));
virtual void init()
// This store is only executed while holding the serial lock, so relaxed
// memory order is sufficient here., memory_order_relaxed);
virtual void fini() { }
static gl_mg o_gl_mg;
// The global lock, write-through TM method.
// Acquires the orec eagerly before the first write, and then writes through.
// Reads abort if the global orec's version number changed or if it is locked.
// Currently, writes require undo-logging to prevent deadlock between the
// serial lock and the global orec (writer txn acquires orec, reader txn
// upgrades to serial and waits for all other txns, writer tries to upgrade to
// serial too but cannot, writer cannot abort either, deadlock). We could
// avoid this if the serial lock would allow us to prevent other threads from
// going to serial mode, but this probably is too much additional complexity
// just to optimize this TM method.
// gtm_thread::shared_state is used to store a transaction's current
// snapshot time (or commit time). The serial lock uses ~0 for inactive
// transactions and 0 for active ones. Thus, we always have a meaningful
// timestamp in shared_state that can be used to implement quiescence-based
// privatization safety. This even holds if a writing transaction has the
// lock bit set in its shared_state because this is fine for both the serial
// lock (the value will be smaller than ~0) and privatization safety (we
// 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 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
static void pre_write(const void *addr, size_t len,
gtm_thread *tx = gtm_thr())
gtm_word v = tx->shared_state.load(memory_order_relaxed);
if (unlikely(!gl_mg::is_locked(v)))
// Check for and handle version number overflow.
if (unlikely(v >= gl_mg::VERSION_MAX))
// This validates that we have a consistent snapshot, which is also
// for making privatization safety work (see the class' comments).
// Note that this check here will be performed by the subsequent CAS
// again, so relaxed memory order is fine.
gtm_word now = o_gl_mg.orec.load(memory_order_relaxed);
if (now != v)
// CAS global orec from our snapshot time to the locked state.
// 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),
// 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.
// Set shared_state to new value.
tx->, memory_order_release);
tx->undolog.log(addr, len);
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
// fence.
// As a result, the data load will synchronize with the release fence
// issued by the transactions whose data updates the data load has read
// from. This forces the orec load to read from a visible sequence of side
// effects that starts with the other updating transaction's store that
// acquired the orec and set it to locked.
// We therefore either read a value with the locked bit set (and restart)
// 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_word l = o_gl_mg.orec.load(memory_order_relaxed);
if (l != tx->shared_state.load(memory_order_relaxed))
template <typename V> static V load(const V* addr, ls_modifier mod)
// Read-for-write should be unlikely, but we need to handle it or will
// break later WaW optimizations.
if (unlikely(mod == RfW))
pre_write(addr, sizeof(V));
return *addr;
if (unlikely(mod == RaW))
return *addr;
// We do not have acquired the orec, so we need to load a value and then
// validate that this was consistent.
// This needs to have acquire memory order (see validate()).
// Alternatively, we can put an acquire fence after the data load but this
// is probably less efficient.
// FIXME We would need an atomic load with acquire memory order here but
// we can't just forge an atomic load for nonatomic data because this
// might not work on all implementations of atomics. However, we need
// the acquire memory order and we can only establish this if we link
// it to the matching release using a reads-from relation between atomic
// loads. Also, the compiler is allowed to optimize nonatomic accesses
// differently than atomic accesses (e.g., if the load would be moved to
// after the fence, we potentially don't synchronize properly anymore).
// Instead of the following, just use an ordinary load followed by an
// acquire fence, and hope that this is good enough for now:
// V v = atomic_load_explicit((atomic<V>*)addr, memory_order_acquire);
V v = *addr;
return v;
template <typename V> static void store(V* addr, const V value,
ls_modifier mod)
if (likely(mod != WaW))
pre_write(addr, sizeof(V));
// FIXME We would need an atomic store here but we can't just forge an
// atomic load for nonatomic data because this might not work on all
// implementations of atomics. However, we need this store to link the
// release fence in pre_write() to the acquire operation in load, which
// is only guaranteed if we have a reads-from relation between atomic
// accesses. Also, the compiler is allowed to optimize nonatomic accesses
// differently than atomic accesses (e.g., if the store would be moved
// to before the release fence in pre_write(), things could go wrong).
// atomic_store_explicit((atomic<V>*)addr, value, memory_order_relaxed);
*addr = value;
static void memtransfer_static(void *dst, const void* src, size_t size,
bool may_overlap, ls_modifier dst_mod, ls_modifier src_mod)
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 (!may_overlap)
::memcpy(dst, src, size);
::memmove(dst, src, size);
if (src_mod != RfW && src_mod != RaW && src_mod != NONTXNAL
&& dst_mod != WaW)
static void memset_static(void *dst, int c, size_t size, ls_modifier mod)
if (mod != WaW)
pre_write(dst, size);
// FIXME We should use atomics here (see store()). Let's just hope that
// memset is good enough.
::memset(dst, c, size);
virtual gtm_restart_reason begin_or_restart()
// We don't need to do anything for nested transactions.
gtm_thread *tx = gtm_thr();
if (tx->parent_txns.size() > 0)
return NO_RESTART;
// Spin until global orec is not locked.
// TODO This is not necessary if there are no pure loads (check txn props).
unsigned i = 0;
gtm_word v;
while (1)
// We need acquire memory order here so that this load will
// synchronize with the store that releases the orec in trycommit().
// In turn, this makes sure that subsequent data loads will read from
// a visible sequence of side effects that starts with the most recent
// store to the data right before the release of the orec.
v = o_gl_mg.orec.load(memory_order_acquire);
if (!gl_mg::is_locked(v))
// TODO need method-specific max spin count
if (++i > gtm_spin_count_var)
// Everything is okay, we have a snapshot time.
// We don't need to enforce any ordering for the following store. There
// are no earlier data loads in this transaction, so the store cannot
// become visible before those (which could lead to the violation of
// privatization safety). The store can become visible after later loads
// but this does not matter because the previous value will have been
// smaller or equal (the serial lock will set shared_state to zero when
// marking the transaction as active, and restarts enforce immediate
// visibility of a smaller or equal value with a barrier (see
// rollback()).
tx->, memory_order_relaxed);
return NO_RESTART;
virtual bool trycommit(gtm_word& priv_time)
gtm_thread* tx = gtm_thr();
gtm_word v = tx->shared_state.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
// location.
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.
v = gl_mg::clear_locked(v) + 1;, 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;
return true;
virtual void rollback(gtm_transaction_cp *cp)
// We don't do anything for rollbacks of nested transactions.
if (cp != 0)
gtm_thread *tx = gtm_thr();
gtm_word v = tx->shared_state.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))
// With our rollback, global time increases.
v = gl_mg::clear_locked(v) + 1;
// 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->, 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)., memory_order_release);
gl_wt_dispatch() : abi_dispatch(false, true, false, false, &o_gl_mg)
{ }
} // anon namespace
static const gl_wt_dispatch o_gl_wt_dispatch;
abi_dispatch *
GTM::dispatch_gl_wt ()
return const_cast<gl_wt_dispatch *>(&o_gl_wt_dispatch);