SDL

1
/*
2
  Simple DirectMedia Layer
3
  Copyright (C) 1997-2024 Sam Lantinga <slouken@libsdl.org>
4

5
  This software is provided 'as-is', without any express or implied
6
  warranty.  In no event will the authors be held liable for any damages
7
  arising from the use of this software.
8

9
  Permission is granted to anyone to use this software for any purpose,
10
  including commercial applications, and to alter it and redistribute it
11
  freely, subject to the following restrictions:
12

13
  1. The origin of this software must not be misrepresented; you must not
14
     claim that you wrote the original software. If you use this software
15
     in a product, an acknowledgment in the product documentation would be
16
     appreciated but is not required.
17
  2. Altered source versions must be plainly marked as such, and must not be
18
     misrepresented as being the original software.
19
  3. This notice may not be removed or altered from any source distribution.
20
*/
21
#include "SDL_internal.h"
22

23
// An implementation of rwlocks using mutexes, condition variables, and atomics.
24

25
#include "SDL_systhread_c.h"
26

27
#include "../generic/SDL_sysrwlock_c.h"
28

29
/* If two implementations are to be compiled into SDL (the active one
30
 * will be chosen at runtime), the function names need to be
31
 * suffixed
32
 */
33
// !!! FIXME: this is quite a tapdance with macros and the build system, maybe we can simplify how we do this. --ryan.
34
#ifndef SDL_THREAD_GENERIC_RWLOCK_SUFFIX
35
#define SDL_CreateRWLock_generic SDL_CreateRWLock
36
#define SDL_DestroyRWLock_generic SDL_DestroyRWLock
37
#define SDL_LockRWLockForReading_generic SDL_LockRWLockForReading
38
#define SDL_LockRWLockForWriting_generic SDL_LockRWLockForWriting
39
#define SDL_UnlockRWLock_generic SDL_UnlockRWLock
40
#endif
41

42
struct SDL_RWLock
43
{
44
#ifdef SDL_THREADS_DISABLED
45
    int unused;
46
#else
47
    SDL_Mutex *lock;
48
    SDL_Condition *condition;
49
    SDL_ThreadID writer_thread;
50
    SDL_AtomicInt reader_count;
51
    SDL_AtomicInt writer_count;
52
#endif
53
};
54

55
SDL_RWLock *SDL_CreateRWLock_generic(void)
56
{
57
    SDL_RWLock *rwlock = (SDL_RWLock *) SDL_calloc(1, sizeof (*rwlock));
58

59
    if (!rwlock) {
60
        return NULL;
61
    }
62

63
#ifndef SDL_THREADS_DISABLED
64
    rwlock->lock = SDL_CreateMutex();
65
    if (!rwlock->lock) {
66
        SDL_free(rwlock);
67
        return NULL;
68
    }
69

70
    rwlock->condition = SDL_CreateCondition();
71
    if (!rwlock->condition) {
72
        SDL_DestroyMutex(rwlock->lock);
73
        SDL_free(rwlock);
74
        return NULL;
75
    }
76

77
    SDL_AtomicSet(&rwlock->reader_count, 0);
78
    SDL_AtomicSet(&rwlock->writer_count, 0);
79
#endif
80

81
    return rwlock;
82
}
83

84
void SDL_DestroyRWLock_generic(SDL_RWLock *rwlock)
85
{
86
    if (rwlock) {
87
#ifndef SDL_THREADS_DISABLED
88
        SDL_DestroyMutex(rwlock->lock);
89
        SDL_DestroyCondition(rwlock->condition);
90
#endif
91
        SDL_free(rwlock);
92
    }
93
}
94

95
void SDL_LockRWLockForReading_generic(SDL_RWLock *rwlock) SDL_NO_THREAD_SAFETY_ANALYSIS  // clang doesn't know about NULL mutexes
96
{
97
#ifndef SDL_THREADS_DISABLED
98
    if (rwlock) {
99
        // !!! FIXME: these don't have to be atomic, we always gate them behind a mutex.
100
        SDL_LockMutex(rwlock->lock);
101
        SDL_assert(SDL_AtomicGet(&rwlock->writer_count) == 0);  // shouldn't be able to grab lock if there's a writer!
102
        SDL_AtomicAdd(&rwlock->reader_count, 1);
103
        SDL_UnlockMutex(rwlock->lock);   // other readers can attempt to share the lock.
104
    }
105
#endif
106
}
107

108
void SDL_LockRWLockForWriting_generic(SDL_RWLock *rwlock) SDL_NO_THREAD_SAFETY_ANALYSIS  // clang doesn't know about NULL mutexes
109
{
110
#ifndef SDL_THREADS_DISABLED
111
    if (rwlock) {
112
        SDL_LockMutex(rwlock->lock);
113
        while (SDL_AtomicGet(&rwlock->reader_count) > 0) {  // while something is holding the shared lock, keep waiting.
114
            SDL_WaitCondition(rwlock->condition, rwlock->lock);  // release the lock and wait for readers holding the shared lock to release it, regrab the lock.
115
        }
116

117
        // we hold the lock!
118
        SDL_AtomicAdd(&rwlock->writer_count, 1);  // we let these be recursive, but the API doesn't require this. It _does_ trust you unlock correctly!
119
    }
120
#endif
121
}
122

123
bool SDL_TryLockRWLockForReading_generic(SDL_RWLock *rwlock)
124
{
125
#ifndef SDL_THREADS_DISABLED
126
    if (rwlock) {
127
        if (!SDL_TryLockMutex(rwlock->lock)) {
128
            // !!! FIXME: there is a small window where a reader has to lock the mutex, and if we hit that, we will return SDL_RWLOCK_TIMEDOUT even though we could have shared the lock.
129
            return false;
130
        }
131

132
        SDL_assert(SDL_AtomicGet(&rwlock->writer_count) == 0);  // shouldn't be able to grab lock if there's a writer!
133
        SDL_AtomicAdd(&rwlock->reader_count, 1);
134
        SDL_UnlockMutex(rwlock->lock);   // other readers can attempt to share the lock.
135
    }
136
#endif
137

138
    return true;
139
}
140

141
#ifndef SDL_THREAD_GENERIC_RWLOCK_SUFFIX
142
SDL_bool SDL_TryLockRWLockForReading(SDL_RWLock *rwlock)
143
{
144
    return SDL_TryLockRWLockForReading_generic(rwlock);
145
}
146
#endif
147

148
bool SDL_TryLockRWLockForWriting_generic(SDL_RWLock *rwlock)
149
{
150
#ifndef SDL_THREADS_DISABLED
151
    if (rwlock) {
152
        if (!SDL_TryLockMutex(rwlock->lock)) {
153
            return false;
154
        }
155

156
        if (SDL_AtomicGet(&rwlock->reader_count) > 0) {  // a reader is using the shared lock, treat it as unavailable.
157
            SDL_UnlockMutex(rwlock->lock);
158
            return false;
159
        }
160

161
        // we hold the lock!
162
        SDL_AtomicAdd(&rwlock->writer_count, 1);  // we let these be recursive, but the API doesn't require this. It _does_ trust you unlock correctly!
163
    }
164
#endif
165

166
    return true;
167
}
168

169
#ifndef SDL_THREAD_GENERIC_RWLOCK_SUFFIX
170
SDL_bool SDL_TryLockRWLockForWriting(SDL_RWLock *rwlock)
171
{
172
    return SDL_TryLockRWLockForWriting_generic(rwlock);
173
}
174
#endif
175

176
void SDL_UnlockRWLock_generic(SDL_RWLock *rwlock) SDL_NO_THREAD_SAFETY_ANALYSIS  // clang doesn't know about NULL mutexes
177
{
178
#ifndef SDL_THREADS_DISABLED
179
    if (rwlock) {
180
        SDL_LockMutex(rwlock->lock);  // recursive lock for writers, readers grab lock to make sure things are sane.
181

182
        if (SDL_AtomicGet(&rwlock->reader_count) > 0) {  // we're a reader
183
            SDL_AtomicAdd(&rwlock->reader_count, -1);
184
            SDL_BroadcastCondition(rwlock->condition);  // alert any pending writers to attempt to try to grab the lock again.
185
        } else if (SDL_AtomicGet(&rwlock->writer_count) > 0) {  // we're a writer
186
            SDL_AtomicAdd(&rwlock->writer_count, -1);
187
            SDL_UnlockMutex(rwlock->lock);  // recursive unlock.
188
        }
189

190
        SDL_UnlockMutex(rwlock->lock);
191
    }
192
#endif
193
}
194

195