668 lines
28 KiB
C++
668 lines
28 KiB
C++
/* Copyright (C) 2012-2022 Free Software Foundation, Inc.
|
|
Contributed by Torvald Riegel <triegel@redhat.com>.
|
|
|
|
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
|
|
<http://www.gnu.org/licenses/>. */
|
|
|
|
#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)
|
|
{
|
|
return (o & INCARNATION_MASK) < INCARNATION_MASK;
|
|
}
|
|
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:
|
|
// http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-115596
|
|
// 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)
|
|
>> L2O_SHIFT;
|
|
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.
|
|
time.store(0, memory_order_relaxed);
|
|
}
|
|
|
|
virtual void fini()
|
|
{
|
|
free(orecs);
|
|
}
|
|
|
|
// 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.
|
|
time.store(0, 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
|
|
{
|
|
protected:
|
|
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);
|
|
do
|
|
{
|
|
// 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)))
|
|
tx->restart(RESTART_LOCKED_WRITE);
|
|
|
|
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)))
|
|
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 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.
|
|
atomic_thread_fence(memory_order_release);
|
|
|
|
// 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;
|
|
}
|
|
oi.advance();
|
|
}
|
|
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))
|
|
tx->restart(RESTART_VALIDATE_READ);
|
|
|
|
// 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->shared_state.store(snapshot, 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);
|
|
do
|
|
{
|
|
// 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)))
|
|
{
|
|
success:
|
|
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;
|
|
}
|
|
else
|
|
{
|
|
// 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)
|
|
tx->restart(RESTART_LOCKED_READ);
|
|
}
|
|
oi.advance();
|
|
}
|
|
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)
|
|
tx->restart(RESTART_VALIDATE_READ);
|
|
}
|
|
}
|
|
|
|
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;
|
|
atomic_thread_fence(memory_order_acquire);
|
|
|
|
// ??? 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;
|
|
}
|
|
|
|
public:
|
|
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);
|
|
else
|
|
::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.
|
|
atomic_thread_fence(memory_order_acquire);
|
|
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)
|
|
return RESTART_INIT_METHOD_GROUP;
|
|
|
|
// 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->shared_state.store(snapshot, 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())
|
|
{
|
|
tx->readlog.clear();
|
|
// 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.
|
|
tx->writelog.clear();
|
|
tx->readlog.clear();
|
|
|
|
// 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)
|
|
return;
|
|
|
|
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))
|
|
i->orec->store(ml_mg::inc_incarnation(i->value),
|
|
memory_order_release);
|
|
else
|
|
{
|
|
// 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().
|
|
atomic_thread_fence(memory_order_release);
|
|
|
|
// We're done, clear the logs.
|
|
tx->writelog.clear();
|
|
tx->readlog.clear();
|
|
}
|
|
|
|
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->shared_state.store(snapshot, 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);
|
|
}
|
|
|
|
CREATE_DISPATCH_METHODS(virtual, )
|
|
CREATE_DISPATCH_METHODS_MEM()
|
|
|
|
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);
|
|
}
|