mirror of
https://github.com/autc04/Retro68.git
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357 lines
15 KiB
C++
357 lines
15 KiB
C++
/* Copyright (C) 2011-2014 Free Software Foundation, Inc.
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Contributed by Torvald Riegel <triegel@redhat.com>.
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This file is part of the GNU Transactional Memory Library (libitm).
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Libitm is free software; you can redistribute it and/or modify it
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under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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Libitm is distributed in the hope that it will be useful, but WITHOUT ANY
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WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
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FOR A PARTICULAR PURPOSE. See the GNU General Public License for
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more details.
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Under Section 7 of GPL version 3, you are granted additional
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permissions described in the GCC Runtime Library Exception, version
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3.1, as published by the Free Software Foundation.
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You should have received a copy of the GNU General Public License and
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a copy of the GCC Runtime Library Exception along with this program;
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see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
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<http://www.gnu.org/licenses/>. */
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#include "libitm_i.h"
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using namespace GTM;
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namespace {
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// This group consists of all TM methods that synchronize via just a single
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// global lock (or ownership record).
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struct gl_mg : public method_group
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{
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static const gtm_word LOCK_BIT = (~(gtm_word)0 >> 1) + 1;
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// We can't use the full bitrange because ~0 in gtm_thread::shared_state has
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// special meaning.
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static const gtm_word VERSION_MAX = (~(gtm_word)0 >> 1) - 1;
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static bool is_locked(gtm_word l) { return l & LOCK_BIT; }
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static gtm_word set_locked(gtm_word l) { return l | LOCK_BIT; }
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static gtm_word clear_locked(gtm_word l) { return l & ~LOCK_BIT; }
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// The global ownership record.
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// No tail-padding necessary (the virtual functions aren't used frequently).
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atomic<gtm_word> orec __attribute__((aligned(HW_CACHELINE_SIZE)));
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virtual void init()
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{
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// This store is only executed while holding the serial lock, so relaxed
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// memory order is sufficient here.
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orec.store(0, memory_order_relaxed);
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}
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virtual void fini() { }
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};
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static gl_mg o_gl_mg;
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// The global lock, write-through TM method.
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// Acquires the orec eagerly before the first write, and then writes through.
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// Reads abort if the global orec's version number changed or if it is locked.
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// Currently, writes require undo-logging to prevent deadlock between the
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// serial lock and the global orec (writer txn acquires orec, reader txn
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// upgrades to serial and waits for all other txns, writer tries to upgrade to
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// serial too but cannot, writer cannot abort either, deadlock). We could
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// avoid this if the serial lock would allow us to prevent other threads from
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// going to serial mode, but this probably is too much additional complexity
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// just to optimize this TM method.
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// gtm_thread::shared_state is used to store a transaction's current
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// snapshot time (or commit time). The serial lock uses ~0 for inactive
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// transactions and 0 for active ones. Thus, we always have a meaningful
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// timestamp in shared_state that can be used to implement quiescence-based
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// privatization safety. This even holds if a writing transaction has the
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// lock bit set in its shared_state because this is fine for both the serial
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// lock (the value will be smaller than ~0) and privatization safety (we
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// validate that no other update transaction comitted before we acquired the
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// orec, so we have the most recent timestamp and no other transaction can
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// commit until we have committed).
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// However, we therefore depend on shared_state not being modified by the
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// serial lock during upgrades to serial mode, which is ensured by
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// gtm_thread::serialirr_mode by not calling gtm_rwlock::write_upgrade_finish
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// before we have committed or rolled back.
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class gl_wt_dispatch : public abi_dispatch
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{
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protected:
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static void pre_write(const void *addr, size_t len,
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gtm_thread *tx = gtm_thr())
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{
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gtm_word v = tx->shared_state.load(memory_order_relaxed);
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if (unlikely(!gl_mg::is_locked(v)))
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{
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// Check for and handle version number overflow.
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if (unlikely(v >= gl_mg::VERSION_MAX))
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tx->restart(RESTART_INIT_METHOD_GROUP);
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// This validates that we have a consistent snapshot, which is also
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// for making privatization safety work (see the class' comments).
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// Note that this check here will be performed by the subsequent CAS
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// again, so relaxed memory order is fine.
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gtm_word now = o_gl_mg.orec.load(memory_order_relaxed);
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if (now != v)
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tx->restart(RESTART_VALIDATE_WRITE);
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// CAS global orec from our snapshot time to the locked state.
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// We need acquire memory order here to synchronize with other
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// (ownership) releases of the orec. We do not need acq_rel order
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// because whenever another thread reads from this CAS'
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// modification, then it will abort anyway and does not rely on
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// any further happens-before relation to be established.
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// Also note that unlike in ml_wt's increase of the global time
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// base (remember that the global orec is used as time base), we do
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// not need require memory order here because we do not need to make
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// prior orec acquisitions visible to other threads that try to
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// extend their snapshot time.
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if (!o_gl_mg.orec.compare_exchange_strong (now, gl_mg::set_locked(now),
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memory_order_acquire))
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tx->restart(RESTART_LOCKED_WRITE);
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// We use an explicit fence here to avoid having to use release
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// memory order for all subsequent data stores. This fence will
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// synchronize with loads of the data with acquire memory order. See
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// validate() for why this is necessary.
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// Adding require memory order to the prior CAS is not sufficient,
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// at least according to the Batty et al. formalization of the
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// memory model.
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atomic_thread_fence(memory_order_release);
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// Set shared_state to new value.
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tx->shared_state.store(gl_mg::set_locked(now), memory_order_release);
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}
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tx->undolog.log(addr, len);
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}
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static void validate(gtm_thread *tx = gtm_thr())
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{
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// Check that snapshot is consistent. We expect the previous data load to
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// have acquire memory order, or be atomic and followed by an acquire
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// fence.
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// As a result, the data load will synchronize with the release fence
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// issued by the transactions whose data updates the data load has read
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// from. This forces the orec load to read from a visible sequence of side
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// effects that starts with the other updating transaction's store that
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// acquired the orec and set it to locked.
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// We therefore either read a value with the locked bit set (and restart)
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// or read an orec value that was written after the data had been written.
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// Either will allow us to detect inconsistent reads because it will have
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// a higher/different value.
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gtm_word l = o_gl_mg.orec.load(memory_order_relaxed);
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if (l != tx->shared_state.load(memory_order_relaxed))
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tx->restart(RESTART_VALIDATE_READ);
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}
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template <typename V> static V load(const V* addr, ls_modifier mod)
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{
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// Read-for-write should be unlikely, but we need to handle it or will
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// break later WaW optimizations.
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if (unlikely(mod == RfW))
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{
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pre_write(addr, sizeof(V));
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return *addr;
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}
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if (unlikely(mod == RaW))
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return *addr;
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// We do not have acquired the orec, so we need to load a value and then
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// validate that this was consistent.
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// This needs to have acquire memory order (see validate()).
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// Alternatively, we can put an acquire fence after the data load but this
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// is probably less efficient.
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// FIXME We would need an atomic load with acquire memory order here but
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// we can't just forge an atomic load for nonatomic data because this
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// might not work on all implementations of atomics. However, we need
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// the acquire memory order and we can only establish this if we link
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// it to the matching release using a reads-from relation between atomic
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// loads. Also, the compiler is allowed to optimize nonatomic accesses
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// differently than atomic accesses (e.g., if the load would be moved to
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// after the fence, we potentially don't synchronize properly anymore).
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// Instead of the following, just use an ordinary load followed by an
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// acquire fence, and hope that this is good enough for now:
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// V v = atomic_load_explicit((atomic<V>*)addr, memory_order_acquire);
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V v = *addr;
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atomic_thread_fence(memory_order_acquire);
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validate();
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return v;
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}
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template <typename V> static void store(V* addr, const V value,
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ls_modifier mod)
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{
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if (likely(mod != WaW))
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pre_write(addr, sizeof(V));
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// FIXME We would need an atomic store here but we can't just forge an
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// atomic load for nonatomic data because this might not work on all
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// implementations of atomics. However, we need this store to link the
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// release fence in pre_write() to the acquire operation in load, which
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// is only guaranteed if we have a reads-from relation between atomic
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// accesses. Also, the compiler is allowed to optimize nonatomic accesses
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// differently than atomic accesses (e.g., if the store would be moved
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// to before the release fence in pre_write(), things could go wrong).
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// atomic_store_explicit((atomic<V>*)addr, value, memory_order_relaxed);
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*addr = value;
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}
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public:
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static void memtransfer_static(void *dst, const void* src, size_t size,
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bool may_overlap, ls_modifier dst_mod, ls_modifier src_mod)
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{
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gtm_thread *tx = gtm_thr();
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if (dst_mod != WaW && dst_mod != NONTXNAL)
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pre_write(dst, size, tx);
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// We need at least undo-logging for an RfW src region because we might
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// subsequently write there with WaW.
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if (src_mod == RfW)
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pre_write(src, size, tx);
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// FIXME We should use atomics here (see store()). Let's just hope that
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// memcpy/memmove are good enough.
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if (!may_overlap)
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::memcpy(dst, src, size);
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else
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::memmove(dst, src, size);
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if (src_mod != RfW && src_mod != RaW && src_mod != NONTXNAL
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&& dst_mod != WaW)
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validate(tx);
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}
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static void memset_static(void *dst, int c, size_t size, ls_modifier mod)
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{
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if (mod != WaW)
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pre_write(dst, size);
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// FIXME We should use atomics here (see store()). Let's just hope that
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// memset is good enough.
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::memset(dst, c, size);
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}
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virtual gtm_restart_reason begin_or_restart()
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{
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// We don't need to do anything for nested transactions.
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gtm_thread *tx = gtm_thr();
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if (tx->parent_txns.size() > 0)
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return NO_RESTART;
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// Spin until global orec is not locked.
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// TODO This is not necessary if there are no pure loads (check txn props).
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unsigned i = 0;
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gtm_word v;
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while (1)
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{
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// We need acquire memory order here so that this load will
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// synchronize with the store that releases the orec in trycommit().
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// In turn, this makes sure that subsequent data loads will read from
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// a visible sequence of side effects that starts with the most recent
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// store to the data right before the release of the orec.
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v = o_gl_mg.orec.load(memory_order_acquire);
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if (!gl_mg::is_locked(v))
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break;
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// TODO need method-specific max spin count
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if (++i > gtm_spin_count_var)
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return RESTART_VALIDATE_READ;
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cpu_relax();
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}
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// Everything is okay, we have a snapshot time.
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// We don't need to enforce any ordering for the following store. There
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// are no earlier data loads in this transaction, so the store cannot
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// become visible before those (which could lead to the violation of
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// privatization safety). The store can become visible after later loads
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// but this does not matter because the previous value will have been
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// smaller or equal (the serial lock will set shared_state to zero when
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// marking the transaction as active, and restarts enforce immediate
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// visibility of a smaller or equal value with a barrier (see
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// rollback()).
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tx->shared_state.store(v, memory_order_relaxed);
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return NO_RESTART;
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}
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virtual bool trycommit(gtm_word& priv_time)
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{
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gtm_thread* tx = gtm_thr();
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gtm_word v = tx->shared_state.load(memory_order_relaxed);
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// Release the orec but do not reset shared_state, which will be modified
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// by the serial lock right after our commit anyway. Also, resetting
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// shared state here would interfere with the serial lock's use of this
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// location.
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if (gl_mg::is_locked(v))
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{
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// Release the global orec, increasing its version number / timestamp.
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// See begin_or_restart() for why we need release memory order here.
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v = gl_mg::clear_locked(v) + 1;
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o_gl_mg.orec.store(v, memory_order_release);
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// Need to ensure privatization safety. Every other transaction must
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// have a snapshot time that is at least as high as our commit time
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// (i.e., our commit must be visible to them).
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priv_time = v;
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}
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return true;
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}
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virtual void rollback(gtm_transaction_cp *cp)
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{
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// We don't do anything for rollbacks of nested transactions.
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if (cp != 0)
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return;
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gtm_thread *tx = gtm_thr();
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gtm_word v = tx->shared_state.load(memory_order_relaxed);
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// Release lock and increment version number to prevent dirty reads.
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// Also reset shared state here, so that begin_or_restart() can expect a
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// value that is correct wrt. privatization safety.
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if (gl_mg::is_locked(v))
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{
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// With our rollback, global time increases.
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v = gl_mg::clear_locked(v) + 1;
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// First reset the timestamp published via shared_state. Release
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// memory order will make this happen after undoing prior data writes.
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// This must also happen before we actually release the global orec
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// next, so that future update transactions in other threads observe
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// a meaningful snapshot time for our transaction; otherwise, they
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// could read a shared_store value with the LOCK_BIT set, which can
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// break privatization safety because it's larger than the actual
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// snapshot time. Note that we only need to consider other update
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// transactions because only those will potentially privatize data.
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tx->shared_state.store(v, memory_order_release);
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// Release the global orec, increasing its version number / timestamp.
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// See begin_or_restart() for why we need release memory order here,
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// and we also need it to make future update transactions read the
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// prior update to shared_state too (update transactions acquire the
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// global orec with acquire memory order).
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o_gl_mg.orec.store(v, memory_order_release);
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}
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}
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CREATE_DISPATCH_METHODS(virtual, )
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CREATE_DISPATCH_METHODS_MEM()
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gl_wt_dispatch() : abi_dispatch(false, true, false, false, 0, &o_gl_mg)
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{ }
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};
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} // anon namespace
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static const gl_wt_dispatch o_gl_wt_dispatch;
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abi_dispatch *
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GTM::dispatch_gl_wt ()
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{
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return const_cast<gl_wt_dispatch *>(&o_gl_wt_dispatch);
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}
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