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/* Copyright (C) 2009-2021 Free Software Foundation, Inc.
Contributed by Richard Henderson <>.
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
<>. */
#ifndef GTM_RWLOCK_H
#define GTM_RWLOCK_H
#include <pthread.h>
#include "local_atomic"
namespace GTM HIDDEN {
struct gtm_thread;
// This datastructure is the blocking, mutex-based side of the Dekker-style
// reader-writer lock used to provide mutual exclusion between active and
// serial transactions. It has similarities to POSIX pthread_rwlock_t except
// that we also provide for upgrading a reader->writer lock, with a
// positive indication of failure (another writer acquired the lock
// before we were able to acquire). While the writer flag (a_writer below) is
// global and protected by the mutex, there are per-transaction reader flags,
// which are stored in a transaction's shared state.
// See libitm's documentation for further details.
// In this implementation, writers are given highest priority access but
// read-to-write upgrades do not have a higher priority than writers.
// Do not change the layout of this class; it must remain a POD type with
// standard layout, and the summary field must be first (i.e., so the
// assembler code can assume that its address is equal to the address of the
// respective instance of the class), and htm_fastpath must be second.
class gtm_rwlock
static const unsigned a_writer = 1; // An active writer.
static const unsigned w_writer = 2; // The w_writers field != 0
static const unsigned w_reader = 4; // The w_readers field != 0
std::atomic<unsigned int> summary; // Bitmask of the above.
// We put the HTM fastpath control variable here so that HTM fastpath
// transactions can check efficiently whether they are allowed to run.
// This must be accessed atomically because threads can load this value
// when they are neither a registered reader nor writer (i.e., when they
// attempt to execute the HTM fastpath).
std::atomic<uint32_t> htm_fastpath;
pthread_mutex_t mutex; // Held if manipulating any field.
pthread_cond_t c_readers; // Readers wait here
pthread_cond_t c_writers; // Writers wait here for writers
pthread_cond_t c_confirmed_writers; // Writers wait here for readers
unsigned int a_readers; // Nr active readers as observed by a writer
unsigned int w_readers; // Nr waiting readers
unsigned int w_writers; // Nr waiting writers
void read_lock (gtm_thread *tx);
void read_unlock (gtm_thread *tx);
void write_lock ();
void write_unlock ();
bool write_upgrade (gtm_thread *tx);
void write_upgrade_finish (gtm_thread *tx);
// Returns true iff there is a concurrent active or waiting writer, or
// htm_fastpath is zero. This is primarily useful for simple HyTM
// approaches, and the values being checked are loaded with
// memory_order_relaxed.
bool htm_fastpath_disabled ()
return (summary.load (memory_order_relaxed) & (a_writer | w_writer))
|| htm_fastpath.load (memory_order_relaxed) == 0;
// This does not need to return an exact value, hence relaxed MO is
// sufficient.
uint32_t get_htm_fastpath ()
return htm_fastpath.load (memory_order_relaxed);
// This must only be called while having acquired the write lock, and other
// threads do not need to load an exact value; hence relaxed MO is
// sufficient.
void set_htm_fastpath (uint32_t val)
{ (val, memory_order_relaxed);
bool write_lock_generic (gtm_thread *tx);
} // namespace GTM
#endif // GTM_RWLOCK_H