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Diffstat (limited to 'gcc-4.9/libitm/method-gl.cc')
| -rw-r--r-- | gcc-4.9/libitm/method-gl.cc | 356 |
1 files changed, 356 insertions, 0 deletions
diff --git a/gcc-4.9/libitm/method-gl.cc b/gcc-4.9/libitm/method-gl.cc new file mode 100644 index 000000000..8e1bf53b7 --- /dev/null +++ b/gcc-4.9/libitm/method-gl.cc @@ -0,0 +1,356 @@ +/* Copyright (C) 2011-2014 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 just a single +// global lock (or ownership record). +struct gl_mg : public method_group +{ + static const gtm_word LOCK_BIT = (~(gtm_word)0 >> 1) + 1; + // We can't use the full bitrange because ~0 in gtm_thread::shared_state has + // special meaning. + static const gtm_word VERSION_MAX = (~(gtm_word)0 >> 1) - 1; + static bool is_locked(gtm_word l) { return l & LOCK_BIT; } + static gtm_word set_locked(gtm_word l) { return l | LOCK_BIT; } + static gtm_word clear_locked(gtm_word l) { return l & ~LOCK_BIT; } + + // The global ownership record. + // No tail-padding necessary (the virtual functions aren't used frequently). + atomic<gtm_word> orec __attribute__((aligned(HW_CACHELINE_SIZE))); + + virtual void init() + { + // This store is only executed while holding the serial lock, so relaxed + // memory order is sufficient here. + orec.store(0, memory_order_relaxed); + } + virtual void fini() { } +}; + +static gl_mg o_gl_mg; + + +// The global lock, write-through TM method. +// Acquires the orec eagerly before the first write, and then writes through. +// Reads abort if the global orec's version number changed or if it is locked. +// Currently, writes require undo-logging to prevent deadlock between the +// serial lock and the global orec (writer txn acquires orec, reader txn +// upgrades to serial and waits for all other txns, writer tries to upgrade to +// serial too but cannot, writer cannot abort either, deadlock). We could +// avoid this if the serial lock would allow us to prevent other threads from +// going to serial mode, but this probably is too much additional complexity +// just to optimize this TM method. +// gtm_thread::shared_state is used to store a transaction's current +// snapshot time (or commit time). The serial lock uses ~0 for inactive +// transactions and 0 for active ones. Thus, we always have a meaningful +// timestamp in shared_state that can be used to implement quiescence-based +// privatization safety. This even holds if a writing transaction has the +// lock bit set in its shared_state because this is fine for both the serial +// lock (the value will be smaller than ~0) and privatization safety (we +// validate that no other update transaction comitted before we acquired the +// orec, so we have the most recent timestamp and no other transaction can +// commit until we have committed). +// However, we therefore depend on shared_state not being modified by the +// serial lock during upgrades to serial mode, which is ensured by +// gtm_thread::serialirr_mode by not calling gtm_rwlock::write_upgrade_finish +// before we have committed or rolled back. +class gl_wt_dispatch : public abi_dispatch +{ +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 acquire memory order here to synchronize with other + // (ownership) releases of the orec. We do not need acq_rel order + // because whenever another thread reads from this CAS' + // modification, then it will abort anyway and does not rely on + // any further happens-before relation to be established. + // Also note that unlike in ml_wt's increase of the global time + // base (remember that the global orec is used as time base), we do + // not need require memory order here because we do not need to make + // prior orec acquisitions visible to other threads that try to + // extend their snapshot time. + if (!o_gl_mg.orec.compare_exchange_strong (now, gl_mg::set_locked(now), + 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 + // validate() for why this is necessary. + // Adding require memory order to the prior CAS is not sufficient, + // at least according to the Batty et al. formalization of the + // memory model. + 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(gtm_thread *tx = gtm_thr()) + { + // Check that snapshot is consistent. We expect the previous data load to + // have acquire memory order, or be atomic and followed by an acquire + // fence. + // As a result, the data load will synchronize with the release fence + // issued by the transactions whose data updates the data load has read + // from. This forces the orec load to read from a visible sequence of side + // effects that starts with the other updating transaction's store that + // acquired the orec and set it to locked. + // We therefore either read a value with the locked bit set (and restart) + // or read an orec value that was written after the data had been written. + // Either will allow us to detect inconsistent reads because it will have + // a higher/different value. + gtm_word l = o_gl_mg.orec.load(memory_order_relaxed); + if (l != tx->shared_state.load(memory_order_relaxed)) + 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; + + // We do not have acquired the orec, so we need to load a value and then + // validate that this was consistent. + // This needs to have acquire memory order (see validate()). + // Alternatively, we can put an acquire fence after the data load but this + // is probably less efficient. + // FIXME We would need an atomic load with acquire memory order here but + // we can't just forge an atomic load for nonatomic data because this + // might not work on all implementations of atomics. However, we need + // the acquire memory order and we can only establish this if we link + // it to the matching release using a reads-from relation between atomic + // loads. Also, the compiler is allowed to optimize nonatomic accesses + // differently than atomic accesses (e.g., if the load would be moved to + // after the fence, we potentially don't synchronize properly anymore). + // Instead of the following, just use an ordinary load followed by an + // acquire fence, and hope that this is good enough for now: + // V v = atomic_load_explicit((atomic<V>*)addr, memory_order_acquire); + V v = *addr; + atomic_thread_fence(memory_order_acquire); + validate(); + 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_thread *tx = gtm_thr(); + if (dst_mod != WaW && dst_mod != NONTXNAL) + pre_write(dst, size, tx); + // We need at least undo-logging for an RfW src region because we might + // subsequently write there with WaW. + if (src_mod == RfW) + pre_write(src, size, tx); + + // FIXME We should use atomics here (see store()). Let's just hope that + // memcpy/memmove are good enough. + if (!may_overlap) + ::memcpy(dst, src, size); + else + ::memmove(dst, src, size); + + if (src_mod != RfW && src_mod != RaW && src_mod != NONTXNAL + && dst_mod != WaW) + validate(tx); + } + + 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); + + // 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); + + // Release lock and increment version number to prevent dirty reads. + // Also reset shared state here, so that begin_or_restart() can expect a + // value that is correct wrt. privatization safety. + if (gl_mg::is_locked(v)) + { + // With our rollback, global time increases. + v = gl_mg::clear_locked(v) + 1; + + // First reset the timestamp published via shared_state. Release + // memory order will make this happen after undoing prior data writes. + // This must also happen before we actually release the global orec + // next, so that future update transactions in other threads observe + // a meaningful snapshot time for our transaction; otherwise, they + // could read a shared_store value with the LOCK_BIT set, which can + // break privatization safety because it's larger than the actual + // snapshot time. Note that we only need to consider other update + // transactions because only those will potentially privatize data. + tx->shared_state.store(v, memory_order_release); + + // Release the global orec, increasing its version number / timestamp. + // See begin_or_restart() for why we need release memory order here, + // and we also need it to make future update transactions read the + // prior update to shared_state too (update transactions acquire the + // global orec with acquire memory order). + o_gl_mg.orec.store(v, memory_order_release); + } + + } + + CREATE_DISPATCH_METHODS(virtual, ) + CREATE_DISPATCH_METHODS_MEM() + + gl_wt_dispatch() : abi_dispatch(false, true, false, false, 0, &o_gl_mg) + { } +}; + +} // anon namespace + +static const gl_wt_dispatch o_gl_wt_dispatch; + +abi_dispatch * +GTM::dispatch_gl_wt () +{ + return const_cast<gl_wt_dispatch *>(&o_gl_wt_dispatch); +} |