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/* Copyright (C) 2011, 2012 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, &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);

}