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/* Copyright (C) 2012-2022 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 multiple locks
// (or ownership records).
struct ml_mg : public method_group
static const gtm_word LOCK_BIT = (~(gtm_word)0 >> 1) + 1;
static const gtm_word INCARNATION_BITS = 3;
static const gtm_word INCARNATION_MASK = 7;
// Maximum time is all bits except the lock bit, the overflow reserve bit,
// and the incarnation bits).
static const gtm_word TIME_MAX = (~(gtm_word)0 >> (2 + INCARNATION_BITS));
// The overflow reserve bit is the MSB of the timestamp part of an orec,
// so we can have TIME_MAX+1 pending timestamp increases before we overflow.
static const gtm_word OVERFLOW_RESERVE = TIME_MAX + 1;
static bool is_locked(gtm_word o) { return o & LOCK_BIT; }
static gtm_word set_locked(gtm_thread *tx)
return ((uintptr_t)tx >> 1) | LOCK_BIT;
// Returns a time that includes the lock bit, which is required by both
// validate() and is_more_recent_or_locked().
static gtm_word get_time(gtm_word o) { return o >> INCARNATION_BITS; }
static gtm_word set_time(gtm_word time) { return time << INCARNATION_BITS; }
static bool is_more_recent_or_locked(gtm_word o, gtm_word than_time)
// LOCK_BIT is the MSB; thus, if O is locked, it is larger than TIME_MAX.
return get_time(o) > than_time;
static bool has_incarnation_left(gtm_word o)
static gtm_word inc_incarnation(gtm_word o) { return o + 1; }
// The shared time base.
atomic<gtm_word> time __attribute__((aligned(HW_CACHELINE_SIZE)));
// The array of ownership records.
atomic<gtm_word>* orecs __attribute__((aligned(HW_CACHELINE_SIZE)));
char tailpadding[HW_CACHELINE_SIZE - sizeof(atomic<gtm_word>*)];
// Location-to-orec mapping. Stripes of 32B mapped to 2^16 orecs using
// multiplicative hashing. See Section 5.2.2 of Torvald Riegel's PhD thesis
// for the background on this choice of hash function and parameters:
// We pick the Mult32 hash because it works well with fewer orecs (i.e.,
// less space overhead and just 32b multiplication).
// We may want to check and potentially change these settings once we get
// better or just more benchmarks.
static const gtm_word L2O_ORECS_BITS = 16;
static const gtm_word L2O_ORECS = 1 << L2O_ORECS_BITS;
// An iterator over the orecs covering the region [addr,addr+len).
struct orec_iterator
static const gtm_word L2O_SHIFT = 5;
static const uint32_t L2O_MULT32 = 81007;
uint32_t mult;
size_t orec;
size_t orec_end;
orec_iterator (const void* addr, size_t len)
uint32_t a = (uintptr_t) addr >> L2O_SHIFT;
uint32_t ae = ((uintptr_t) addr + len + (1 << L2O_SHIFT) - 1)
mult = a * L2O_MULT32;
orec = mult >> (32 - L2O_ORECS_BITS);
// We can't really avoid this second multiplication unless we use a
// branch instead or know more about the alignment of addr. (We often
// know len at compile time because of instantiations of functions
// such as _ITM_RU* for accesses of specific lengths.
orec_end = (ae * L2O_MULT32) >> (32 - L2O_ORECS_BITS);
size_t get() { return orec; }
void advance()
// We cannot simply increment orec because L2O_MULT32 is larger than
// 1 << (32 - L2O_ORECS_BITS), and thus an increase of the stripe (i.e.,
// addr >> L2O_SHIFT) could increase the resulting orec index by more
// than one; with the current parameters, we would roughly acquire a
// fourth more orecs than necessary for regions covering more than orec.
// Keeping mult around as extra state shouldn't matter much.
mult += L2O_MULT32;
orec = mult >> (32 - L2O_ORECS_BITS);
bool reached_end() { return orec == orec_end; }
virtual void init()
// We assume that an atomic<gtm_word> is backed by just a gtm_word, so
// starting with zeroed memory is fine.
orecs = (atomic<gtm_word>*) xcalloc(
sizeof(atomic<gtm_word>) * L2O_ORECS, true);
// This store is only executed while holding the serial lock, so relaxed
// memory order is sufficient here., memory_order_relaxed);
virtual void fini()
// We only re-initialize when our time base overflows. Thus, only reset
// the time base and the orecs but do not re-allocate the orec array.
virtual void reinit()
// This store is only executed while holding the serial lock, so relaxed
// memory order is sufficient here. Same holds for the memset., memory_order_relaxed);
// The memset below isn't strictly kosher because it bypasses
// the non-trivial assignment operator defined by std::atomic. Using
// a local void* is enough to prevent GCC from warning for this.
void *p = orecs;
memset(p, 0, sizeof(atomic<gtm_word>) * L2O_ORECS);
static ml_mg o_ml_mg;
// The multiple lock, write-through TM method.
// Maps each memory location to one of the orecs in the orec array, and then
// acquires the associated orec eagerly before writing through.
// Writes require undo-logging because we are dealing with several locks/orecs
// and need to resolve deadlocks if necessary by aborting one of the
// transactions.
// Reads do time-based validation with snapshot time extensions. Incarnation
// numbers are used to decrease contention on the time base (with those,
// aborted transactions do not need to acquire a new version number for the
// data that has been previously written in the transaction and needs to be
// rolled back).
// 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.
class ml_wt_dispatch : public abi_dispatch
static void pre_write(gtm_thread *tx, const void *addr, size_t len)
gtm_word snapshot = tx->shared_state.load(memory_order_relaxed);
gtm_word locked_by_tx = ml_mg::set_locked(tx);
// Lock all orecs that cover the region.
ml_mg::orec_iterator oi(addr, len);
// Load the orec. Relaxed memory order is sufficient here because
// either we have acquired the orec or we will try to acquire it with
// a CAS with stronger memory order.
gtm_word o = o_ml_mg.orecs[oi.get()].load(memory_order_relaxed);
// Check whether we have acquired the orec already.
if (likely (locked_by_tx != o))
// If not, acquire. Make sure that our snapshot time is larger or
// equal than the orec's version to avoid masking invalidations of
// our snapshot with our own writes.
if (unlikely (ml_mg::is_locked(o)))
if (unlikely (ml_mg::get_time(o) > snapshot))
// We only need to extend the snapshot if we have indeed read
// from this orec before. Given that we are an update
// transaction, we will have to extend anyway during commit.
// ??? Scan the read log instead, aborting if we have read
// from data covered by this orec before?
snapshot = extend(tx);
// 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.
if (unlikely (!o_ml_mg.orecs[oi.get()].compare_exchange_strong(
o, locked_by_tx, memory_order_acquire)))
// 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 post_load() 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.
// We log the previous value here to be able to use incarnation
// numbers when we have to roll back.
// ??? Reserve capacity early to avoid capacity checks here?
gtm_rwlog_entry *e = tx->writelog.push();
e->orec = o_ml_mg.orecs + oi.get();
e->value = o;
while (!oi.reached_end());
// Do undo logging. We do not know which region prior writes logged
// (even if orecs have been acquired), so just log everything.
tx->undolog.log(addr, len);
static void pre_write(const void *addr, size_t len)
gtm_thread *tx = gtm_thr();
pre_write(tx, addr, len);
// Returns true iff all the orecs in our read log still have the same time
// or have been locked by the transaction itself.
static bool validate(gtm_thread *tx)
gtm_word locked_by_tx = ml_mg::set_locked(tx);
// ??? This might get called from pre_load() via extend(). In that case,
// we don't really need to check the new entries that pre_load() is
// adding. Stop earlier?
for (gtm_rwlog_entry *i = tx->readlog.begin(), *ie = tx->readlog.end();
i != ie; i++)
// Relaxed memory order is sufficient here because we do not need to
// establish any new synchronizes-with relationships. We only need
// to read a value that is as least as current as enforced by the
// callers: extend() loads global time with acquire, and trycommit()
// increments global time with acquire. Therefore, we will see the
// most recent orec updates before the global time that we load.
gtm_word o = i->orec->load(memory_order_relaxed);
// We compare only the time stamp and the lock bit here. We know that
// we have read only committed data before, so we can ignore
// intermediate yet rolled-back updates presented by the incarnation
// number bits.
if (ml_mg::get_time(o) != ml_mg::get_time(i->value)
&& o != locked_by_tx)
return false;
return true;
// Tries to extend the snapshot to a more recent time. Returns the new
// snapshot time and updates TX->SHARED_STATE. If the snapshot cannot be
// extended to the current global time, TX is restarted.
static gtm_word extend(gtm_thread *tx)
// We read global time here, even if this isn't strictly necessary
// because we could just return the maximum of the timestamps that
// validate sees. However, the potential cache miss on global time is
// probably a reasonable price to pay for avoiding unnecessary extensions
// in the future.
// We need acquire memory oder because we have to synchronize with the
// increment of global time by update transactions, whose lock
// acquisitions we have to observe (also see trycommit()).
gtm_word snapshot = o_ml_mg.time.load(memory_order_acquire);
if (!validate(tx))
// Update our public snapshot time. Probably useful to decrease waiting
// due to quiescence-based privatization safety.
// Use release memory order to establish synchronizes-with with the
// privatizers; prior data loads should happen before the privatizers
// potentially modify anything.
tx->, memory_order_release);
return snapshot;
// First pass over orecs. Load and check all orecs that cover the region.
// Write to read log, extend snapshot time if necessary.
static gtm_rwlog_entry* pre_load(gtm_thread *tx, const void* addr,
size_t len)
// Don't obtain an iterator yet because the log might get resized.
size_t log_start = tx->readlog.size();
gtm_word snapshot = tx->shared_state.load(memory_order_relaxed);
gtm_word locked_by_tx = ml_mg::set_locked(tx);
ml_mg::orec_iterator oi(addr, len);
// 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.
gtm_word o = o_ml_mg.orecs[oi.get()].load(memory_order_acquire);
if (likely (!ml_mg::is_more_recent_or_locked(o, snapshot)))
gtm_rwlog_entry *e = tx->readlog.push();
e->orec = o_ml_mg.orecs + oi.get();
e->value = o;
else if (!ml_mg::is_locked(o))
// We cannot read this part of the region because it has been
// updated more recently than our snapshot time. If we can extend
// our snapshot, then we can read.
snapshot = extend(tx);
goto success;
// If the orec is locked by us, just skip it because we can just
// read from it. Otherwise, restart the transaction.
if (o != locked_by_tx)
while (!oi.reached_end());
return &tx->readlog[log_start];
// Second pass over orecs, verifying that the we had a consistent read.
// Restart the transaction if any of the orecs is locked by another
// transaction.
static void post_load(gtm_thread *tx, gtm_rwlog_entry* log)
for (gtm_rwlog_entry *end = tx->readlog.end(); log != end; log++)
// Check that the 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.
// Also note that differently to validate(), we compare the raw value
// of the orec here, including incarnation numbers. We must prevent
// returning uncommitted data from loads (whereas when validating, we
// already performed a consistent load).
gtm_word o = log->orec->load(memory_order_relaxed);
if (log->value != o)
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;
// ??? Optimize for RaR?
gtm_thread *tx = gtm_thr();
gtm_rwlog_entry* log = pre_load(tx, addr, sizeof(V));
// Load the data.
// This needs to have acquire memory order (see post_load()).
// 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;
// ??? Retry the whole load if it wasn't consistent?
post_load(tx, log);
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_rwlog_entry* log = 0;
gtm_thread *tx = 0;
if (src_mod == RfW)
tx = gtm_thr();
pre_write(tx, src, size);
else if (src_mod != RaW && src_mod != NONTXNAL)
tx = gtm_thr();
log = pre_load(tx, src, size);
// ??? Optimize for RaR?
if (dst_mod != NONTXNAL && dst_mod != WaW)
if (src_mod != RfW && (src_mod == RaW || src_mod == NONTXNAL))
tx = gtm_thr();
pre_write(tx, dst, size);
// 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);
// ??? Retry the whole memtransfer if it wasn't consistent?
if (src_mod != RfW && src_mod != RaW && src_mod != NONTXNAL)
// See load() for why we need the acquire fence here.
post_load(tx, log);
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;
// Read the current time, which becomes our snapshot time.
// Use acquire memory oder so that we see the lock acquisitions by update
// transcations that incremented the global time (see trycommit()).
gtm_word snapshot = o_ml_mg.time.load(memory_order_acquire);
// Re-initialize method group on time overflow.
if (snapshot >= o_ml_mg.TIME_MAX)
// 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();
// If we haven't updated anything, we can commit.
if (!tx->writelog.size())
// We still need to ensure privatization safety, unfortunately. While
// we cannot have privatized anything by ourselves (because we are not
// an update transaction), we can have observed the commits of
// another update transaction that privatized something. Because any
// commit happens before ensuring privatization, our snapshot and
// commit can thus have happened before ensuring privatization safety
// for this commit/snapshot time. Therefore, before we can return to
// nontransactional code that might use the privatized data, we must
// ensure privatization safety for our snapshot time.
// This still seems to be better than not allowing use of the
// snapshot time before privatization safety has been ensured because
// we at least can run transactions such as this one, and in the
// meantime the transaction producing this commit time might have
// finished ensuring privatization safety for it.
priv_time = tx->shared_state.load(memory_order_relaxed);
return true;
// Get a commit time.
// Overflow of o_ml_mg.time is prevented in begin_or_restart().
// We need acq_rel here because (1) the acquire part is required for our
// own subsequent call to validate(), and the release part is necessary to
// make other threads' validate() work as explained there and in extend().
gtm_word ct = o_ml_mg.time.fetch_add(1, memory_order_acq_rel) + 1;
// Extend our snapshot time to at least our commit time.
// Note that we do not need to validate if our snapshot time is right
// before the commit time because we are never sharing the same commit
// time with other transactions.
// No need to reset shared_state, which will be modified by the serial
// lock right after our commit anyway.
gtm_word snapshot = tx->shared_state.load(memory_order_relaxed);
if (snapshot < ct - 1 && !validate(tx))
return false;
// Release orecs.
// See pre_load() / post_load() for why we need release memory order.
// ??? Can we use a release fence and relaxed stores?
gtm_word v = ml_mg::set_time(ct);
for (gtm_rwlog_entry *i = tx->writelog.begin(), *ie = tx->writelog.end();
i != ie; i++)
i->orec->store(v, memory_order_release);
// We're done, clear the logs.
// 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 = ct;
return true;
virtual void rollback(gtm_transaction_cp *cp)
// We don't do anything for rollbacks of nested transactions.
// ??? We could release locks here if we snapshot writelog size. readlog
// is similar. This is just a performance optimization though. Nested
// aborts should be rather infrequent, so the additional save/restore
// overhead for the checkpoints could be higher.
if (cp != 0)
gtm_thread *tx = gtm_thr();
gtm_word overflow_value = 0;
// Release orecs.
for (gtm_rwlog_entry *i = tx->writelog.begin(), *ie = tx->writelog.end();
i != ie; i++)
// If possible, just increase the incarnation number.
// See pre_load() / post_load() for why we need release memory order.
// ??? Can we use a release fence and relaxed stores? (Same below.)
if (ml_mg::has_incarnation_left(i->value))
// We have an incarnation overflow. Acquire a new timestamp, and
// use it from now on as value for each orec whose incarnation
// number cannot be increased.
// Overflow of o_ml_mg.time is prevented in begin_or_restart().
// See pre_load() / post_load() for why we need release memory
// order.
if (!overflow_value)
// Release memory order is sufficient but required here.
// In contrast to the increment in trycommit(), we need release
// for the same reason but do not need the acquire because we
// do not validate subsequently.
overflow_value = ml_mg::set_time(
o_ml_mg.time.fetch_add(1, memory_order_release) + 1);
i->orec->store(overflow_value, memory_order_release);
// We need this release fence to ensure that privatizers see the
// rolled-back original state (not any uncommitted values) when they read
// the new snapshot time that we write in begin_or_restart().
// We're done, clear the logs.
virtual bool snapshot_most_recent()
// This is the same code as in extend() except that we do not restart
// on failure but simply return the result, and that we don't validate
// if our snapshot is already most recent.
gtm_thread* tx = gtm_thr();
gtm_word snapshot = o_ml_mg.time.load(memory_order_acquire);
if (snapshot == tx->shared_state.load(memory_order_relaxed))
return true;
if (!validate(tx))
return false;
// Update our public snapshot time. Necessary so that we do not prevent
// other transactions from ensuring privatization safety.
tx->, memory_order_release);
return true;
virtual bool supports(unsigned number_of_threads)
// Each txn can commit and fail and rollback once before checking for
// overflow, so this bounds the number of threads that we can support.
// In practice, this won't be a problem but we check it anyway so that
// we never break in the occasional weird situation.
return (number_of_threads * 2 <= ml_mg::OVERFLOW_RESERVE);
ml_wt_dispatch() : abi_dispatch(false, true, false, false, 0, &o_ml_mg)
{ }
} // anon namespace
static const ml_wt_dispatch o_ml_wt_dispatch;
abi_dispatch *
GTM::dispatch_ml_wt ()
return const_cast<ml_wt_dispatch *>(&o_ml_wt_dispatch);