diff --git a/libitm/ChangeLog b/libitm/ChangeLog index e44057d38d3..4dbb1e85ecd 100644 --- a/libitm/ChangeLog +++ b/libitm/ChangeLog @@ -1,3 +1,13 @@ +2012-02-14 Torvald Riegel + + * libitm_i.h (GTM::gtm_rwlog_entry): New. + (GTM::gtm_thread): Add read and write logs. + (GTM::dispatch_ml_wt): Declare. + * retry.cc (parse_default_method): Support ml_wt. + * method-ml.cc: New file. + * Makefile.am: Add method-ml.cc. + * Makefile.in: Regenerate. + 2012-02-14 Torvald Riegel * dispatch.h (GTM::abi_dispatch::supports): New. diff --git a/libitm/Makefile.am b/libitm/Makefile.am index f1c351c21d5..e754ccc7704 100644 --- a/libitm/Makefile.am +++ b/libitm/Makefile.am @@ -60,7 +60,7 @@ libitm_la_SOURCES = \ aatree.cc alloc.cc alloc_c.cc alloc_cpp.cc barrier.cc beginend.cc \ clone.cc eh_cpp.cc local.cc \ query.cc retry.cc rwlock.cc useraction.cc util.cc \ - sjlj.S tls.cc method-serial.cc method-gl.cc + sjlj.S tls.cc method-serial.cc method-gl.cc method-ml.cc if ARCH_ARM libitm_la_SOURCES += hwcap.cc diff --git a/libitm/Makefile.in b/libitm/Makefile.in index 22baeb8de85..66aacaba716 100644 --- a/libitm/Makefile.in +++ b/libitm/Makefile.in @@ -100,15 +100,15 @@ libitm_la_LIBADD = am__libitm_la_SOURCES_DIST = aatree.cc alloc.cc alloc_c.cc \ alloc_cpp.cc barrier.cc beginend.cc clone.cc eh_cpp.cc \ local.cc query.cc retry.cc rwlock.cc useraction.cc util.cc \ - sjlj.S tls.cc method-serial.cc method-gl.cc hwcap.cc \ - x86_sse.cc x86_avx.cc futex.cc + sjlj.S tls.cc method-serial.cc method-gl.cc method-ml.cc \ + hwcap.cc x86_sse.cc x86_avx.cc futex.cc @ARCH_ARM_TRUE@am__objects_1 = hwcap.lo @ARCH_X86_TRUE@am__objects_2 = x86_sse.lo x86_avx.lo @ARCH_FUTEX_TRUE@am__objects_3 = futex.lo am_libitm_la_OBJECTS = aatree.lo alloc.lo alloc_c.lo alloc_cpp.lo \ barrier.lo beginend.lo clone.lo eh_cpp.lo local.lo query.lo \ retry.lo rwlock.lo useraction.lo util.lo sjlj.lo tls.lo \ - method-serial.lo method-gl.lo $(am__objects_1) \ + method-serial.lo method-gl.lo method-ml.lo $(am__objects_1) \ $(am__objects_2) $(am__objects_3) libitm_la_OBJECTS = $(am_libitm_la_OBJECTS) DEFAULT_INCLUDES = -I.@am__isrc@ @@ -379,8 +379,8 @@ libitm_la_LDFLAGS = $(libitm_version_info) $(libitm_version_script) libitm_la_SOURCES = aatree.cc alloc.cc alloc_c.cc alloc_cpp.cc \ barrier.cc beginend.cc clone.cc eh_cpp.cc local.cc query.cc \ retry.cc rwlock.cc useraction.cc util.cc sjlj.S tls.cc \ - method-serial.cc method-gl.cc $(am__append_1) $(am__append_2) \ - $(am__append_3) + method-serial.cc method-gl.cc method-ml.cc $(am__append_1) \ + $(am__append_2) $(am__append_3) # Automake Documentation: # If your package has Texinfo files in many directories, you can use the @@ -512,6 +512,7 @@ distclean-compile: @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/hwcap.Plo@am__quote@ @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/local.Plo@am__quote@ @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/method-gl.Plo@am__quote@ +@AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/method-ml.Plo@am__quote@ @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/method-serial.Plo@am__quote@ @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/query.Plo@am__quote@ @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/retry.Plo@am__quote@ diff --git a/libitm/libitm_i.h b/libitm/libitm_i.h index bf17086ef2e..e826abd0656 100644 --- a/libitm/libitm_i.h +++ b/libitm/libitm_i.h @@ -145,6 +145,13 @@ struct gtm_undolog void rollback (gtm_thread* tx, size_t until_size = 0); }; +// An entry of a read or write log. Used by multi-lock TM methods. +struct gtm_rwlog_entry +{ + atomic *orec; + gtm_word value; +}; + // Contains all thread-specific data required by the entire library. // This includes all data relevant to a single transaction. Because most // thread-specific data is about the current transaction, we also refer to @@ -174,6 +181,10 @@ struct gtm_thread // Data used by local.c for the undo log for both local and shared memory. gtm_undolog undolog; + // Read and write logs. Used by multi-lock TM methods. + vector readlog; + vector writelog; + // Data used by alloc.c for the malloc/free undo log. aa_tree alloc_actions; @@ -320,6 +331,7 @@ extern abi_dispatch *dispatch_serial(); extern abi_dispatch *dispatch_serialirr(); extern abi_dispatch *dispatch_serialirr_onwrite(); extern abi_dispatch *dispatch_gl_wt(); +extern abi_dispatch *dispatch_ml_wt(); extern gtm_cacheline_mask gtm_mask_stack(gtm_cacheline *, gtm_cacheline_mask); diff --git a/libitm/method-ml.cc b/libitm/method-ml.cc new file mode 100644 index 00000000000..88455e8f85e --- /dev/null +++ b/libitm/method-ml.cc @@ -0,0 +1,605 @@ +/* Copyright (C) 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 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 time __attribute__((aligned(HW_CACHELINE_SIZE))); + + // The array of ownership records. + atomic* orecs __attribute__((aligned(HW_CACHELINE_SIZE))); + char tailpadding[HW_CACHELINE_SIZE - sizeof(atomic*)]; + + // Location-to-orec mapping. Stripes of 16B mapped to 2^19 orecs. + static const gtm_word L2O_ORECS = 1 << 19; + static const gtm_word L2O_SHIFT = 4; + static size_t get_orec(const void* addr) + { + return ((uintptr_t)addr >> L2O_SHIFT) & (L2O_ORECS - 1); + } + static size_t get_next_orec(size_t orec) + { + return (orec + 1) & (L2O_ORECS - 1); + } + // Returns the next orec after the region. + static size_t get_orec_end(const void* addr, size_t len) + { + return (((uintptr_t)addr + len + (1 << L2O_SHIFT) - 1) >> L2O_SHIFT) + & (L2O_ORECS - 1); + } + + virtual void init() + { + // We assume that an atomic is backed by just a gtm_word, so + // starting with zeroed memory is fine. + orecs = (atomic*) xcalloc( + sizeof(atomic) * 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); + memset(orecs, 0, sizeof(atomic) * 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. + size_t orec = ml_mg::get_orec(addr); + size_t orec_end = ml_mg::get_orec_end(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[orec].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[orec].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 + orec; + e->value = o; + } + orec = o_ml_mg.get_next_orec(orec); + } + while (orec != orec_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); + + size_t orec = ml_mg::get_orec(addr); + size_t orec_end = ml_mg::get_orec_end(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[orec].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 + orec; + 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); + } + orec = o_ml_mg.get_next_orec(orec); + } + while (orec != orec_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 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*)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 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) + { + 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(); + 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 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, &o_ml_mg) + { } +}; + +} // anon namespace + +static const ml_wt_dispatch o_ml_wt_dispatch; + +abi_dispatch * +GTM::dispatch_ml_wt () +{ + return const_cast(&o_ml_wt_dispatch); +} diff --git a/libitm/retry.cc b/libitm/retry.cc index 761a066e834..d59c1834ef0 100644 --- a/libitm/retry.cc +++ b/libitm/retry.cc @@ -199,6 +199,11 @@ parse_default_method() disp = GTM::dispatch_gl_wt(); env += 5; } + else if (strncmp(env, "ml_wt", 5) == 0) + { + disp = GTM::dispatch_ml_wt(); + env += 5; + } else goto unknown;