| /* Copyright (C) 2011-2022 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/>. */ |
| |
| #ifndef GTM_RWLOCK_H |
| #define GTM_RWLOCK_H |
| |
| #include "local_atomic" |
| #include "common.h" |
| |
| namespace GTM HIDDEN { |
| |
| struct gtm_thread; |
| |
| // This datastructure is the blocking, futex-based version of the Dekker-style |
| // reader-writer lock used to provide mutual exclusion between active and |
| // serial transactions. |
| // 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 writers 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 |
| { |
| std::atomic<int> writers; // Writers' futex. |
| // 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; |
| // TODO Put these futexes on different cachelines? (writers and htm_fastpath |
| // should remain on the same cacheline. |
| std::atomic<int> writer_readers;// A confirmed writer waits here for readers. |
| std::atomic<int> readers; // Readers wait here for writers (iff true). |
| |
| public: |
| gtm_rwlock() : writers(0), htm_fastpath(0), writer_readers(0), readers(0) |
| { } |
| |
| 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 writers.load (memory_order_relaxed) != 0 |
| || 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) |
| { |
| htm_fastpath.store (val, memory_order_relaxed); |
| } |
| |
| protected: |
| bool write_lock_generic (gtm_thread *tx); |
| }; |
| |
| } // namespace GTM |
| |
| #endif // GTM_RWLOCK_H |