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Diffstat (limited to 'libitm/method-gl.cc')
| -rw-r--r-- | libitm/method-gl.cc | 303 |
1 files changed, 303 insertions, 0 deletions
diff --git a/libitm/method-gl.cc b/libitm/method-gl.cc new file mode 100644 index 00000000000..60c26626779 --- /dev/null +++ b/libitm/method-gl.cc @@ -0,0 +1,303 @@ +/* Copyright (C) 2011 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. + gtm_word orec; + virtual void init() + { + orec = 0; + } + 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 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(); + if (unlikely(!gl_mg::is_locked(tx->shared_state))) + { + // Check for and handle version number overflow. + if (unlikely(tx->shared_state >= gl_mg::VERSION_MAX)) + tx->restart(RESTART_INIT_METHOD_GROUP); + + // CAS global orec from our snapshot time to the locked state. + // This validates that we have a consistent snapshot, which is also + // for making privatization safety work (see the class' comments). + gtm_word now = o_gl_mg.orec; + if (now != tx->shared_state) + tx->restart(RESTART_VALIDATE_WRITE); + if (__sync_val_compare_and_swap(&o_gl_mg.orec, now, + gl_mg::set_locked(now)) != now) + tx->restart(RESTART_LOCKED_WRITE); + + // Set shared_state to new value. The CAS is a full barrier, so the + // acquisition of the global orec is visible before this store here, + // and the store will not be visible before earlier data loads, which + // is required to correctly ensure privatization safety (see + // begin_and_restart() and release_orec() for further comments). + tx->shared_state = gl_mg::set_locked(now); + } + + // TODO Ensure that this gets inlined: Use internal log interface and LTO. + GTM_LB(addr, len); + } + + static void validate() + { + // Check that snapshot is consistent. The barrier ensures that this + // happens after previous data loads. + atomic_read_barrier(); + gtm_thread *tx = gtm_thr(); + gtm_word l = o_gl_mg.orec; + if (l != tx->shared_state) + 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; + } + V v = *addr; + if (likely(mod != RaW)) + validate(); + return v; + } + + template <typename V> static void store(V* addr, const V value, + ls_modifier mod) + { + if (unlikely(mod != WaW)) + pre_write(addr, sizeof(V)); + *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); + + 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); + ::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). + gtm_word v; + unsigned i = 0; + while (gl_mg::is_locked(v = o_gl_mg.orec)) + { + // TODO need method-specific max spin count + if (++i > gtm_spin_count_var) return RESTART_VALIDATE_READ; + cpu_relax(); + } + // This barrier ensures that we have read the global orec before later + // data loads. + atomic_read_barrier(); + + // 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 + // release_orec()). + tx->shared_state = v; + return NO_RESTART; + } + + virtual bool trycommit(gtm_word& priv_time) + { + gtm_thread* tx = gtm_thr(); + gtm_word v = tx->shared_state; + + // 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; + + // 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. + // TODO replace with C++0x-style atomics (a release in this case) + atomic_write_barrier(); + v = gl_mg::clear_locked(v) + 1; + o_gl_mg.orec = v; + + // 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; + // 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. + if (v == ~(typeof v)0) + v = o_gl_mg.orec; + + // 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. + // TODO replace with C++0x-style atomics (a release in this case) + atomic_write_barrier(); + v = gl_mg::clear_locked(v) + 1; + o_gl_mg.orec = v; + + // 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 (tx->shared_state != ~(typeof tx->shared_state)0) + tx->shared_state = v; + + // 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). + __sync_synchronize(); + } + + } + + 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<gl_wt_dispatch *>(&o_gl_wt_dispatch); +} |
