1 /* Copyright (C) 2011-2014 Free Software Foundation, Inc. 2 Contributed by Torvald Riegel <triegel@redhat.com>. 3 4 This file is part of the GNU Transactional Memory Library (libitm). 5 6 Libitm is free software; you can redistribute it and/or modify it 7 under the terms of the GNU General Public License as published by 8 the Free Software Foundation; either version 3 of the License, or 9 (at your option) any later version. 10 11 Libitm is distributed in the hope that it will be useful, but WITHOUT ANY 12 WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS 13 FOR A PARTICULAR PURPOSE. See the GNU General Public License for 14 more details. 15 16 Under Section 7 of GPL version 3, you are granted additional 17 permissions described in the GCC Runtime Library Exception, version 18 3.1, as published by the Free Software Foundation. 19 20 You should have received a copy of the GNU General Public License and 21 a copy of the GCC Runtime Library Exception along with this program; 22 see the files COPYING3 and COPYING.RUNTIME respectively. If not, see 23 <http://www.gnu.org/licenses/>. */ 24 25 #ifndef GTM_RWLOCK_H 26 #define GTM_RWLOCK_H 27 28 #include "local_atomic" 29 #include "common.h" 30 31 namespace GTM HIDDEN { 32 33 struct gtm_thread; 34 35 // This datastructure is the blocking, futex-based version of the Dekker-style 36 // reader-writer lock used to provide mutual exclusion between active and 37 // serial transactions. 38 // See libitm's documentation for further details. 39 // 40 // In this implementation, writers are given highest priority access but 41 // read-to-write upgrades do not have a higher priority than writers. 42 // 43 // Do not change the layout of this class; it must remain a POD type with 44 // standard layout, and the WRITERS field must be first (i.e., so the 45 // assembler code can assume that its address is equal to the address of the 46 // respective instance of the class). 47 48 class gtm_rwlock 49 { 50 // TODO Put futexes on different cachelines? 51 std::atomic<int> writers; // Writers' futex. 52 std::atomic<int> writer_readers;// A confirmed writer waits here for readers. 53 std::atomic<int> readers; // Readers wait here for writers (iff true). 54 55 public: gtm_rwlock()56 gtm_rwlock() : writers(0), writer_readers(0), readers(0) {}; 57 58 void read_lock (gtm_thread *tx); 59 void read_unlock (gtm_thread *tx); 60 61 void write_lock (); 62 void write_unlock (); 63 64 bool write_upgrade (gtm_thread *tx); 65 void write_upgrade_finish (gtm_thread *tx); 66 67 // Returns true iff there is a concurrent active or waiting writer. 68 // This is primarily useful for simple HyTM approaches, and the value being 69 // checked is loaded with memory_order_relaxed. is_write_locked()70 bool is_write_locked() 71 { 72 return writers.load (memory_order_relaxed) != 0; 73 } 74 75 protected: 76 bool write_lock_generic (gtm_thread *tx); 77 }; 78 79 } // namespace GTM 80 81 #endif // GTM_RWLOCK_H 82