1 /* Copyright (C) 2009-2020 Free Software Foundation, Inc. 2 Contributed by Richard Henderson <rth@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 <pthread.h> 29 #include "local_atomic" 30 31 namespace GTM HIDDEN { 32 33 struct gtm_thread; 34 35 // This datastructure is the blocking, mutex-based side of the Dekker-style 36 // reader-writer lock used to provide mutual exclusion between active and 37 // serial transactions. It has similarities to POSIX pthread_rwlock_t except 38 // that we also provide for upgrading a reader->writer lock, with a 39 // positive indication of failure (another writer acquired the lock 40 // before we were able to acquire). While the writer flag (a_writer below) is 41 // global and protected by the mutex, there are per-transaction reader flags, 42 // which are stored in a transaction's shared state. 43 // See libitm's documentation for further details. 44 // 45 // In this implementation, writers are given highest priority access but 46 // read-to-write upgrades do not have a higher priority than writers. 47 // 48 // Do not change the layout of this class; it must remain a POD type with 49 // standard layout, and the summary field must be first (i.e., so the 50 // assembler code can assume that its address is equal to the address of the 51 // respective instance of the class), and htm_fastpath must be second. 52 53 class gtm_rwlock 54 { 55 static const unsigned a_writer = 1; // An active writer. 56 static const unsigned w_writer = 2; // The w_writers field != 0 57 static const unsigned w_reader = 4; // The w_readers field != 0 58 59 std::atomic<unsigned int> summary; // Bitmask of the above. 60 61 // We put the HTM fastpath control variable here so that HTM fastpath 62 // transactions can check efficiently whether they are allowed to run. 63 // This must be accessed atomically because threads can load this value 64 // when they are neither a registered reader nor writer (i.e., when they 65 // attempt to execute the HTM fastpath). 66 std::atomic<uint32_t> htm_fastpath; 67 68 pthread_mutex_t mutex; // Held if manipulating any field. 69 pthread_cond_t c_readers; // Readers wait here 70 pthread_cond_t c_writers; // Writers wait here for writers 71 pthread_cond_t c_confirmed_writers; // Writers wait here for readers 72 73 unsigned int a_readers; // Nr active readers as observed by a writer 74 unsigned int w_readers; // Nr waiting readers 75 unsigned int w_writers; // Nr waiting writers 76 77 public: 78 gtm_rwlock(); 79 ~gtm_rwlock(); 80 81 void read_lock (gtm_thread *tx); 82 void read_unlock (gtm_thread *tx); 83 84 void write_lock (); 85 void write_unlock (); 86 87 bool write_upgrade (gtm_thread *tx); 88 void write_upgrade_finish (gtm_thread *tx); 89 90 // Returns true iff there is a concurrent active or waiting writer, or 91 // htm_fastpath is zero. This is primarily useful for simple HyTM 92 // approaches, and the values being checked are loaded with 93 // memory_order_relaxed. htm_fastpath_disabled()94 bool htm_fastpath_disabled () 95 { 96 return (summary.load (memory_order_relaxed) & (a_writer | w_writer)) 97 || htm_fastpath.load (memory_order_relaxed) == 0; 98 } 99 100 // This does not need to return an exact value, hence relaxed MO is 101 // sufficient. get_htm_fastpath()102 uint32_t get_htm_fastpath () 103 { 104 return htm_fastpath.load (memory_order_relaxed); 105 } 106 // This must only be called while having acquired the write lock, and other 107 // threads do not need to load an exact value; hence relaxed MO is 108 // sufficient. set_htm_fastpath(uint32_t val)109 void set_htm_fastpath (uint32_t val) 110 { 111 htm_fastpath.store (val, memory_order_relaxed); 112 } 113 114 protected: 115 bool write_lock_generic (gtm_thread *tx); 116 }; 117 118 } // namespace GTM 119 120 #endif // GTM_RWLOCK_H 121