MySQL  8.0.19
Source Code Documentation
thr_rwlock_bits.h
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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 
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  */
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_THR_RWLOCK_BITS_H */
rw_pr_lock_t::no_active_readers
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
rw_pr_lock_t::active_readers
unsigned int active_readers
Number of active readers.
Definition: thr_rwlock_bits.h:106
rw_pr_lock_t::lock
native_mutex_t lock
Lock which protects the structure.
Definition: thr_rwlock_bits.h:99
my_thread_bits.h
native_cond_t
pthread_cond_t native_cond_t
Definition: thr_cond_bits.h:45
thr_mutex_bits.h
rw_pr_lock_t::writers_waiting_readers
unsigned int writers_waiting_readers
Number of writers waiting for readers to go away.
Definition: thr_rwlock_bits.h:108
rw_pr_lock_t
Portable implementation of special type of read-write locks.
Definition: thr_rwlock_bits.h:94
native_rw_lock_t
pthread_rwlock_t native_rw_lock_t
Definition: thr_rwlock_bits.h:60
native_mutex_t
pthread_mutex_t native_mutex_t
Definition: thr_mutex_bits.h:54
my_thread_t
pthread_t my_thread_t
Definition: my_thread_bits.h:47
rw_pr_lock_t::active_writer
bool active_writer
Indicates whether there is an active writer.
Definition: thr_rwlock_bits.h:110
thr_cond_bits.h
rw_pr_lock_t::writer_thread
my_thread_t writer_thread
Thread holding wr-lock (for debug purposes only).
Definition: thr_rwlock_bits.h:112