/* 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. atomic orec; virtual void init() { // This store is only executed while holding the serial lock, so relaxed // memory order is sufficient here. orec.store(0, memory_order_relaxed); } virtual void fini() { } }; // TODO cacheline padding 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 cannot use this method for a serial transaction // (because shared_state needs to remain at ~0) and we have to be careful // when switching to serial mode (see the special handling in trycommit() and // rollback()). // ??? This sharing adds some complexity wrt. serial mode. Just use a separate // state variable? class gl_wt_dispatch : public abi_dispatch { protected: 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)) tx->restart(RESTART_INIT_METHOD_GROUP); // 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) tx->restart(RESTART_VALIDATE_WRITE); // CAS global orec from our snapshot time to the locked state. // We need acq_rel memory order here to synchronize with other loads // and modifications of orec. if (!o_gl_mg.orec.compare_exchange_strong (now, gl_mg::set_locked(now), memory_order_acq_rel)) tx->restart(RESTART_LOCKED_WRITE); // We use an explicit fence here to avoid having to use release // memory order for all subsequent data stores. This fence will // synchronize with loads of the data with acquire memory order. See // validate() for why this is necessary. atomic_thread_fence(memory_order_release); // Set shared_state to new value. tx->shared_state.store(gl_mg::set_locked(now), memory_order_release); } tx->undolog.log(addr, len); } static void validate() { // 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_thread *tx = gtm_thr(); gtm_word l = o_gl_mg.orec.load(memory_order_relaxed); if (l != tx->shared_state.load(memory_order_relaxed)) tx->restart(RESTART_VALIDATE_READ); } template 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*)addr, memory_order_acquire); V v = *addr; atomic_thread_fence(memory_order_acquire); validate(); return v; } template 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*)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) { if ((dst_mod != WaW && src_mod != RaW) && (dst_mod != NONTXNAL || src_mod == RfW)) pre_write(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); if (src_mod != RfW && src_mod != RaW && src_mod != NONTXNAL && dst_mod != WaW) validate(); } 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)) break; // TODO need method-specific max spin count if (++i > gtm_spin_count_var) return RESTART_VALIDATE_READ; cpu_relax(); } // 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->shared_state.store(v, 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); // Special case: If shared_state is ~0, then we have acquired the // serial lock (tx->state is not updated yet). In this case, the previous // value isn't available anymore, so grab it from the global lock, which // must have a meaningful value because no other transactions are active // anymore. In particular, if it is locked, then we are an update // transaction, which is all we care about for commit. if (v == ~(typeof v)0) v = o_gl_mg.orec.load(memory_order_relaxed); // Release the orec but do not reset shared_state, which will be modified // by the serial lock right after our commit anyway. Also, resetting // shared state here would interfere with the serial lock's use of this // 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; o_gl_mg.orec.store(v, memory_order_release); // Need to ensure privatization safety. Every other transaction must // have a snapshot time that is at least as high as our commit time // (i.e., our commit must be visible to them). priv_time = v; } return true; } virtual void rollback(gtm_transaction_cp *cp) { // We don't do anything for rollbacks of nested transactions. if (cp != 0) return; gtm_thread *tx = gtm_thr(); gtm_word v = tx->shared_state.load(memory_order_relaxed); // Special case: If shared_state is ~0, then we have acquired the // serial lock (tx->state is not updated yet). In this case, the previous // value isn't available anymore, so grab it from the global lock, which // must have a meaningful value because no other transactions are active // anymore. In particular, if it is locked, then we are an update // transaction, which is all we care about for rollback. bool is_serial = v == ~(typeof v)0; if (is_serial) v = o_gl_mg.orec.load(memory_order_relaxed); // Release lock and increment version number to prevent dirty reads. // Also reset shared state here, so that begin_or_restart() can expect a // value that is correct wrt. privatization safety. if (gl_mg::is_locked(v)) { // Release the global orec, increasing its version number / timestamp. // See begin_or_restart() for why we need release memory order here. v = gl_mg::clear_locked(v) + 1; o_gl_mg.orec.store(v, memory_order_release); // Also reset the timestamp published via shared_state. // Special case: Only do this if we are not a serial transaction // because otherwise, we would interfere with the serial lock. if (!is_serial) tx->shared_state.store(v, memory_order_release); // We need a store-load barrier after this store to prevent it // from becoming visible after later data loads because the // previous value of shared_state has been higher than the actual // snapshot time (the lock bit had been set), which could break // privatization safety. We do not need a barrier before this // store (see pre_write() for an explanation). // ??? What is the precise reasoning in the C++11 model? atomic_thread_fence(memory_order_seq_cst); } } CREATE_DISPATCH_METHODS(virtual, ) CREATE_DISPATCH_METHODS_MEM() 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(&o_gl_wt_dispatch); }