MySQL 8.0.30
Source Code Documentation
thr_rwlock_bits.h
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22
23#ifndef COMPONENTS_SERVICES_BITS_THR_RWLOCK_BITS_H
24#define COMPONENTS_SERVICES_BITS_THR_RWLOCK_BITS_H
25
26/**
27 @file mysql/components/services/bits/thr_rwlock_bits.h
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
53
54#ifdef _WIN32
55struct native_rw_lock_t {
56 SRWLOCK srwlock; /* native reader writer lock */
57 BOOL have_exclusive_srwlock; /* used for unlock */
58};
59#else
60typedef 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 often 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
95 /**
96 Lock which protects the structure.
97 Also held for the duration of wr-lock.
98 */
100 /**
101 Condition variable which is used to wake-up
102 writers waiting for readers to go away.
103 */
105 /** Number of active readers. */
106 unsigned int active_readers;
107 /** Number of writers waiting for readers to go away. */
109 /** Indicates whether there is an active writer. */
111 /** Thread holding wr-lock (for debug purposes only). */
113};
114
115#endif /* COMPONENTS_SERVICES_BITS_THR_RWLOCK_BITS_H */
Types to make different thread packages compatible.
pthread_t my_thread_t
Definition: my_thread_bits.h:47
Portable implementation of special type of read-write locks.
Definition: thr_rwlock_bits.h:94
unsigned int writers_waiting_readers
Number of writers waiting for readers to go away.
Definition: thr_rwlock_bits.h:108
native_cond_t no_active_readers
Condition variable which is used to wake-up writers waiting for readers to go away.
Definition: thr_rwlock_bits.h:104
my_thread_t writer_thread
Thread holding wr-lock (for debug purposes only).
Definition: thr_rwlock_bits.h:112
unsigned int active_readers
Number of active readers.
Definition: thr_rwlock_bits.h:106
native_mutex_t lock
Lock which protects the structure.
Definition: thr_rwlock_bits.h:99
bool active_writer
Indicates whether there is an active writer.
Definition: thr_rwlock_bits.h:110
MySQL condition variable implementation.
pthread_cond_t native_cond_t
Definition: thr_cond_bits.h:45
ABI for thd_mutex.
pthread_mutex_t native_mutex_t
Definition: thr_mutex_bits.h:54
pthread_rwlock_t native_rw_lock_t
Definition: thr_rwlock_bits.h:60