1 /* Copyright (c) 2014, 2020, Oracle and/or its affiliates. All rights reserved. 2 3 This program is free software; you can redistribute it and/or modify 4 it under the terms of the GNU General Public License, version 2.0, 5 as published by the Free Software Foundation. 6 7 This program is also distributed with certain software (including 8 but not limited to OpenSSL) that is licensed under separate terms, 9 as designated in a particular file or component or in included license 10 documentation. The authors of MySQL hereby grant you an additional 11 permission to link the program and your derivative works with the 12 separately licensed software that they have included with MySQL. 13 14 This program is distributed in the hope that it will be useful, 15 but WITHOUT ANY WARRANTY; without even the implied warranty of 16 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 17 GNU General Public License, version 2.0, for more details. 18 19 You should have received a copy of the GNU General Public License 20 along with this program; if not, write to the Free Software 21 Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA */ 22 23 #ifndef COMPONENTS_SERVICES_THR_RWLOCK_BITS_H 24 #define COMPONENTS_SERVICES_THR_RWLOCK_BITS_H 25 26 /** 27 @file 28 MySQL rwlock ABI. 29 30 There are two "layers": 31 1) native_rw_*() 32 Functions that map directly down to OS primitives. 33 Windows - SRWLock 34 Other OSes - pthread 35 2) mysql_rw*() 36 Functions that include Performance Schema instrumentation. 37 See include/mysql/psi/mysql_thread.h 38 39 This file also includes rw_pr_*(), which implements a special 40 version of rwlocks that prefer readers. The P_S version of these 41 are mysql_prlock_*() - see include/mysql/psi/mysql_thread.h 42 */ 43 44 #include <stddef.h> 45 #include <sys/types.h> 46 #ifdef _WIN32 47 #include <windows.h> 48 #endif 49 50 #include <mysql/components/services/my_thread_bits.h> 51 #include <mysql/components/services/thr_cond_bits.h> 52 #include <mysql/components/services/thr_mutex_bits.h> 53 54 #ifdef _WIN32 55 struct native_rw_lock_t { 56 SRWLOCK srwlock; /* native reader writer lock */ 57 BOOL have_exclusive_srwlock; /* used for unlock */ 58 }; 59 #else 60 typedef pthread_rwlock_t native_rw_lock_t; 61 #endif 62 63 /** 64 Portable implementation of special type of read-write locks. 65 66 These locks have two properties which are unusual for rwlocks: 67 1) They "prefer readers" in the sense that they do not allow 68 situations in which rwlock is rd-locked and there is a 69 pending rd-lock which is blocked (e.g. due to pending 70 request for wr-lock). 71 This is a stronger guarantee than one which is provided for 72 PTHREAD_RWLOCK_PREFER_READER_NP rwlocks in Linux. 73 MDL subsystem deadlock detector relies on this property for 74 its correctness. 75 2) They are optimized for uncontended wr-lock/unlock case. 76 This is scenario in which they are most oftenly used 77 within MDL subsystem. Optimizing for it gives significant 78 performance improvements in some of tests involving many 79 connections. 80 81 Another important requirement imposed on this type of rwlock 82 by the MDL subsystem is that it should be OK to destroy rwlock 83 object which is in unlocked state even though some threads might 84 have not yet fully left unlock operation for it (of course there 85 is an external guarantee that no thread will try to lock rwlock 86 which is destroyed). 87 Putting it another way the unlock operation should not access 88 rwlock data after changing its state to unlocked. 89 90 TODO/FIXME: We should consider alleviating this requirement as 91 it blocks us from doing certain performance optimizations. 92 */ 93 94 struct rw_pr_lock_t { 95 /** 96 Lock which protects the structure. 97 Also held for the duration of wr-lock. 98 */ 99 native_mutex_t lock; 100 /** 101 Condition variable which is used to wake-up 102 writers waiting for readers to go away. 103 */ 104 native_cond_t no_active_readers; 105 /** Number of active readers. */ 106 unsigned int active_readers; 107 /** Number of writers waiting for readers to go away. */ 108 unsigned int writers_waiting_readers; 109 /** Indicates whether there is an active writer. */ 110 bool active_writer; 111 /** Thread holding wr-lock (for debug purposes only). */ 112 my_thread_t writer_thread; 113 }; 114 115 #endif /* COMPONENTS_SERVICES_THR_RWLOCK_BITS_H */ 116