121 lines
4.5 KiB
C++
121 lines
4.5 KiB
C++
/* Copyright (C) 2009-2021 Free Software Foundation, Inc.
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Contributed by Richard Henderson <rth@redhat.com>.
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This file is part of the GNU Transactional Memory Library (libitm).
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Libitm is free software; you can redistribute it and/or modify it
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under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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Libitm is distributed in the hope that it will be useful, but WITHOUT ANY
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WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
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FOR A PARTICULAR PURPOSE. See the GNU General Public License for
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more details.
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Under Section 7 of GPL version 3, you are granted additional
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permissions described in the GCC Runtime Library Exception, version
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3.1, as published by the Free Software Foundation.
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You should have received a copy of the GNU General Public License and
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a copy of the GCC Runtime Library Exception along with this program;
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see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
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<http://www.gnu.org/licenses/>. */
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#ifndef GTM_RWLOCK_H
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#define GTM_RWLOCK_H
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#include <pthread.h>
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#include "local_atomic"
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namespace GTM HIDDEN {
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struct gtm_thread;
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// This datastructure is the blocking, mutex-based side of the Dekker-style
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// reader-writer lock used to provide mutual exclusion between active and
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// serial transactions. It has similarities to POSIX pthread_rwlock_t except
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// that we also provide for upgrading a reader->writer lock, with a
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// positive indication of failure (another writer acquired the lock
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// before we were able to acquire). While the writer flag (a_writer below) is
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// global and protected by the mutex, there are per-transaction reader flags,
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// which are stored in a transaction's shared state.
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// See libitm's documentation for further details.
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//
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// In this implementation, writers are given highest priority access but
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// read-to-write upgrades do not have a higher priority than writers.
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//
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// Do not change the layout of this class; it must remain a POD type with
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// standard layout, and the summary field must be first (i.e., so the
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// assembler code can assume that its address is equal to the address of the
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// respective instance of the class), and htm_fastpath must be second.
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class gtm_rwlock
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{
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static const unsigned a_writer = 1; // An active writer.
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static const unsigned w_writer = 2; // The w_writers field != 0
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static const unsigned w_reader = 4; // The w_readers field != 0
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std::atomic<unsigned int> summary; // Bitmask of the above.
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// We put the HTM fastpath control variable here so that HTM fastpath
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// transactions can check efficiently whether they are allowed to run.
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// This must be accessed atomically because threads can load this value
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// when they are neither a registered reader nor writer (i.e., when they
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// attempt to execute the HTM fastpath).
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std::atomic<uint32_t> htm_fastpath;
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pthread_mutex_t mutex; // Held if manipulating any field.
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pthread_cond_t c_readers; // Readers wait here
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pthread_cond_t c_writers; // Writers wait here for writers
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pthread_cond_t c_confirmed_writers; // Writers wait here for readers
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unsigned int a_readers; // Nr active readers as observed by a writer
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unsigned int w_readers; // Nr waiting readers
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unsigned int w_writers; // Nr waiting writers
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public:
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gtm_rwlock();
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~gtm_rwlock();
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void read_lock (gtm_thread *tx);
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void read_unlock (gtm_thread *tx);
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void write_lock ();
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void write_unlock ();
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bool write_upgrade (gtm_thread *tx);
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void write_upgrade_finish (gtm_thread *tx);
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// Returns true iff there is a concurrent active or waiting writer, or
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// htm_fastpath is zero. This is primarily useful for simple HyTM
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// approaches, and the values being checked are loaded with
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// memory_order_relaxed.
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bool htm_fastpath_disabled ()
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{
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return (summary.load (memory_order_relaxed) & (a_writer | w_writer))
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|| htm_fastpath.load (memory_order_relaxed) == 0;
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}
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// This does not need to return an exact value, hence relaxed MO is
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// sufficient.
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uint32_t get_htm_fastpath ()
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{
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return htm_fastpath.load (memory_order_relaxed);
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}
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// This must only be called while having acquired the write lock, and other
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// threads do not need to load an exact value; hence relaxed MO is
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// sufficient.
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void set_htm_fastpath (uint32_t val)
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{
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htm_fastpath.store (val, memory_order_relaxed);
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}
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protected:
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bool write_lock_generic (gtm_thread *tx);
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};
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} // namespace GTM
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#endif // GTM_RWLOCK_H
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