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1 /*
2 ** 2007 August 28
3 **
4 ** The author disclaims copyright to this source code. In place of
5 ** a legal notice, here is a blessing:
6 **
7 ** May you do good and not evil.
8 ** May you find forgiveness for yourself and forgive others.
9 ** May you share freely, never taking more than you give.
11 *************************************************************************
12 ** This file contains the C functions that implement mutexes for pthreads
14 #include "sqliteInt.h"
17 ** The code in this file is only used if we are compiling threadsafe
18 ** under unix with pthreads.
20 ** Note that this implementation requires a version of pthreads that
21 ** supports recursive mutexes.
23 #ifdef SQLITE_MUTEX_PTHREADS
25 #include <pthread.h>
28 ** The sqlite3_mutex.id, sqlite3_mutex.nRef, and sqlite3_mutex.owner fields
29 ** are necessary under two condidtions: (1) Debug builds and (2) using
30 ** home-grown mutexes. Encapsulate these conditions into a single #define.
32 #if defined(SQLITE_DEBUG) || defined(SQLITE_HOMEGROWN_RECURSIVE_MUTEX)
33 # define SQLITE_MUTEX_NREF 1
34 #else
35 # define SQLITE_MUTEX_NREF 0
36 #endif
39 ** Each recursive mutex is an instance of the following structure.
41 struct sqlite3_mutex {
42 pthread_mutex_t mutex; /* Mutex controlling the lock */
43 #if SQLITE_MUTEX_NREF
44 int id; /* Mutex type */
45 volatile int nRef; /* Number of entrances */
46 volatile pthread_t owner; /* Thread that is within this mutex */
47 int trace; /* True to trace changes */
48 #endif
50 #if SQLITE_MUTEX_NREF
51 #define SQLITE3_MUTEX_INITIALIZER { PTHREAD_MUTEX_INITIALIZER, 0, 0, (pthread_t)0, 0 }
52 #else
53 #define SQLITE3_MUTEX_INITIALIZER { PTHREAD_MUTEX_INITIALIZER }
54 #endif
57 ** The sqlite3_mutex_held() and sqlite3_mutex_notheld() routine are
58 ** intended for use only inside assert() statements. On some platforms,
59 ** there might be race conditions that can cause these routines to
60 ** deliver incorrect results. In particular, if pthread_equal() is
61 ** not an atomic operation, then these routines might delivery
62 ** incorrect results. On most platforms, pthread_equal() is a
63 ** comparison of two integers and is therefore atomic. But we are
64 ** told that HPUX is not such a platform. If so, then these routines
65 ** will not always work correctly on HPUX.
67 ** On those platforms where pthread_equal() is not atomic, SQLite
68 ** should be compiled without -DSQLITE_DEBUG and with -DNDEBUG to
69 ** make sure no assert() statements are evaluated and hence these
70 ** routines are never called.
72 #if !defined(NDEBUG) || defined(SQLITE_DEBUG)
73 static int pthreadMutexHeld(sqlite3_mutex *p){
74 return (p->nRef!=0 && pthread_equal(p->owner, pthread_self()));
76 static int pthreadMutexNotheld(sqlite3_mutex *p){
77 return p->nRef==0 || pthread_equal(p->owner, pthread_self())==0;
79 #endif
82 ** Initialize and deinitialize the mutex subsystem.
84 static int pthreadMutexInit(void){ return SQLITE_OK; }
85 static int pthreadMutexEnd(void){ return SQLITE_OK; }
88 ** The sqlite3_mutex_alloc() routine allocates a new
89 ** mutex and returns a pointer to it. If it returns NULL
90 ** that means that a mutex could not be allocated. SQLite
91 ** will unwind its stack and return an error. The argument
92 ** to sqlite3_mutex_alloc() is one of these integer constants:
94 ** <ul>
95 ** <li> SQLITE_MUTEX_FAST
96 ** <li> SQLITE_MUTEX_RECURSIVE
97 ** <li> SQLITE_MUTEX_STATIC_MASTER
98 ** <li> SQLITE_MUTEX_STATIC_MEM
99 ** <li> SQLITE_MUTEX_STATIC_OPEN
100 ** <li> SQLITE_MUTEX_STATIC_PRNG
101 ** <li> SQLITE_MUTEX_STATIC_LRU
102 ** <li> SQLITE_MUTEX_STATIC_PMEM
103 ** <li> SQLITE_MUTEX_STATIC_APP1
104 ** <li> SQLITE_MUTEX_STATIC_APP2
105 ** <li> SQLITE_MUTEX_STATIC_APP3
106 ** </ul>
108 ** The first two constants cause sqlite3_mutex_alloc() to create
109 ** a new mutex. The new mutex is recursive when SQLITE_MUTEX_RECURSIVE
110 ** is used but not necessarily so when SQLITE_MUTEX_FAST is used.
111 ** The mutex implementation does not need to make a distinction
112 ** between SQLITE_MUTEX_RECURSIVE and SQLITE_MUTEX_FAST if it does
113 ** not want to. But SQLite will only request a recursive mutex in
114 ** cases where it really needs one. If a faster non-recursive mutex
115 ** implementation is available on the host platform, the mutex subsystem
116 ** might return such a mutex in response to SQLITE_MUTEX_FAST.
118 ** The other allowed parameters to sqlite3_mutex_alloc() each return
119 ** a pointer to a static preexisting mutex. Six static mutexes are
120 ** used by the current version of SQLite. Future versions of SQLite
121 ** may add additional static mutexes. Static mutexes are for internal
122 ** use by SQLite only. Applications that use SQLite mutexes should
123 ** use only the dynamic mutexes returned by SQLITE_MUTEX_FAST or
124 ** SQLITE_MUTEX_RECURSIVE.
126 ** Note that if one of the dynamic mutex parameters (SQLITE_MUTEX_FAST
127 ** or SQLITE_MUTEX_RECURSIVE) is used then sqlite3_mutex_alloc()
128 ** returns a different mutex on every call. But for the static
129 ** mutex types, the same mutex is returned on every call that has
130 ** the same type number.
132 static sqlite3_mutex *pthreadMutexAlloc(int iType){
133 static sqlite3_mutex staticMutexes[] = {
134 SQLITE3_MUTEX_INITIALIZER,
135 SQLITE3_MUTEX_INITIALIZER,
136 SQLITE3_MUTEX_INITIALIZER,
137 SQLITE3_MUTEX_INITIALIZER,
138 SQLITE3_MUTEX_INITIALIZER,
139 SQLITE3_MUTEX_INITIALIZER,
140 SQLITE3_MUTEX_INITIALIZER,
141 SQLITE3_MUTEX_INITIALIZER,
142 SQLITE3_MUTEX_INITIALIZER
144 sqlite3_mutex *p;
145 switch( iType ){
146 case SQLITE_MUTEX_RECURSIVE: {
147 p = sqlite3MallocZero( sizeof(*p) );
148 if( p ){
149 #ifdef SQLITE_HOMEGROWN_RECURSIVE_MUTEX
150 /* If recursive mutexes are not available, we will have to
151 ** build our own. See below. */
152 pthread_mutex_init(&p->mutex, 0);
153 #else
154 /* Use a recursive mutex if it is available */
155 pthread_mutexattr_t recursiveAttr;
156 pthread_mutexattr_init(&recursiveAttr);
157 pthread_mutexattr_settype(&recursiveAttr, PTHREAD_MUTEX_RECURSIVE);
158 pthread_mutex_init(&p->mutex, &recursiveAttr);
159 pthread_mutexattr_destroy(&recursiveAttr);
160 #endif
161 #if SQLITE_MUTEX_NREF
162 p->id = iType;
163 #endif
165 break;
167 case SQLITE_MUTEX_FAST: {
168 p = sqlite3MallocZero( sizeof(*p) );
169 if( p ){
170 #if SQLITE_MUTEX_NREF
171 p->id = iType;
172 #endif
173 pthread_mutex_init(&p->mutex, 0);
175 break;
177 default: {
178 assert( iType-2 >= 0 );
179 assert( iType-2 < ArraySize(staticMutexes) );
180 p = &staticMutexes[iType-2];
181 #if SQLITE_MUTEX_NREF
182 p->id = iType;
183 #endif
184 break;
187 return p;
192 ** This routine deallocates a previously
193 ** allocated mutex. SQLite is careful to deallocate every
194 ** mutex that it allocates.
196 static void pthreadMutexFree(sqlite3_mutex *p){
197 assert( p->nRef==0 );
198 assert( p->id==SQLITE_MUTEX_FAST || p->id==SQLITE_MUTEX_RECURSIVE );
199 pthread_mutex_destroy(&p->mutex);
200 sqlite3_free(p);
204 ** The sqlite3_mutex_enter() and sqlite3_mutex_try() routines attempt
205 ** to enter a mutex. If another thread is already within the mutex,
206 ** sqlite3_mutex_enter() will block and sqlite3_mutex_try() will return
207 ** SQLITE_BUSY. The sqlite3_mutex_try() interface returns SQLITE_OK
208 ** upon successful entry. Mutexes created using SQLITE_MUTEX_RECURSIVE can
209 ** be entered multiple times by the same thread. In such cases the,
210 ** mutex must be exited an equal number of times before another thread
211 ** can enter. If the same thread tries to enter any other kind of mutex
212 ** more than once, the behavior is undefined.
214 static void pthreadMutexEnter(sqlite3_mutex *p){
215 assert( p->id==SQLITE_MUTEX_RECURSIVE || pthreadMutexNotheld(p) );
217 #ifdef SQLITE_HOMEGROWN_RECURSIVE_MUTEX
218 /* If recursive mutexes are not available, then we have to grow
219 ** our own. This implementation assumes that pthread_equal()
220 ** is atomic - that it cannot be deceived into thinking self
221 ** and p->owner are equal if p->owner changes between two values
222 ** that are not equal to self while the comparison is taking place.
223 ** This implementation also assumes a coherent cache - that
224 ** separate processes cannot read different values from the same
225 ** address at the same time. If either of these two conditions
226 ** are not met, then the mutexes will fail and problems will result.
229 pthread_t self = pthread_self();
230 if( p->nRef>0 && pthread_equal(p->owner, self) ){
231 p->nRef++;
232 }else{
233 pthread_mutex_lock(&p->mutex);
234 assert( p->nRef==0 );
235 p->owner = self;
236 p->nRef = 1;
239 #else
240 /* Use the built-in recursive mutexes if they are available.
242 pthread_mutex_lock(&p->mutex);
243 #if SQLITE_MUTEX_NREF
244 assert( p->nRef>0 || p->owner==0 );
245 p->owner = pthread_self();
246 p->nRef++;
247 #endif
248 #endif
250 #ifdef SQLITE_DEBUG
251 if( p->trace ){
252 printf("enter mutex %p (%d) with nRef=%d\n", p, p->trace, p->nRef);
254 #endif
256 static int pthreadMutexTry(sqlite3_mutex *p){
257 int rc;
258 assert( p->id==SQLITE_MUTEX_RECURSIVE || pthreadMutexNotheld(p) );
260 #ifdef SQLITE_HOMEGROWN_RECURSIVE_MUTEX
261 /* If recursive mutexes are not available, then we have to grow
262 ** our own. This implementation assumes that pthread_equal()
263 ** is atomic - that it cannot be deceived into thinking self
264 ** and p->owner are equal if p->owner changes between two values
265 ** that are not equal to self while the comparison is taking place.
266 ** This implementation also assumes a coherent cache - that
267 ** separate processes cannot read different values from the same
268 ** address at the same time. If either of these two conditions
269 ** are not met, then the mutexes will fail and problems will result.
272 pthread_t self = pthread_self();
273 if( p->nRef>0 && pthread_equal(p->owner, self) ){
274 p->nRef++;
275 rc = SQLITE_OK;
276 }else if( pthread_mutex_trylock(&p->mutex)==0 ){
277 assert( p->nRef==0 );
278 p->owner = self;
279 p->nRef = 1;
280 rc = SQLITE_OK;
281 }else{
282 rc = SQLITE_BUSY;
285 #else
286 /* Use the built-in recursive mutexes if they are available.
288 if( pthread_mutex_trylock(&p->mutex)==0 ){
289 #if SQLITE_MUTEX_NREF
290 p->owner = pthread_self();
291 p->nRef++;
292 #endif
293 rc = SQLITE_OK;
294 }else{
295 rc = SQLITE_BUSY;
297 #endif
299 #ifdef SQLITE_DEBUG
300 if( rc==SQLITE_OK && p->trace ){
301 printf("enter mutex %p (%d) with nRef=%d\n", p, p->trace, p->nRef);
303 #endif
304 return rc;
308 ** The sqlite3_mutex_leave() routine exits a mutex that was
309 ** previously entered by the same thread. The behavior
310 ** is undefined if the mutex is not currently entered or
311 ** is not currently allocated. SQLite will never do either.
313 static void pthreadMutexLeave(sqlite3_mutex *p){
314 assert( pthreadMutexHeld(p) );
315 #if SQLITE_MUTEX_NREF
316 p->nRef--;
317 if( p->nRef==0 ) p->owner = 0;
318 #endif
319 assert( p->nRef==0 || p->id==SQLITE_MUTEX_RECURSIVE );
321 #ifdef SQLITE_HOMEGROWN_RECURSIVE_MUTEX
322 if( p->nRef==0 ){
323 pthread_mutex_unlock(&p->mutex);
325 #else
326 pthread_mutex_unlock(&p->mutex);
327 #endif
329 #ifdef SQLITE_DEBUG
330 if( p->trace ){
331 printf("leave mutex %p (%d) with nRef=%d\n", p, p->trace, p->nRef);
333 #endif
336 sqlite3_mutex_methods const *sqlite3DefaultMutex(void){
337 static const sqlite3_mutex_methods sMutex = {
338 pthreadMutexInit,
339 pthreadMutexEnd,
340 pthreadMutexAlloc,
341 pthreadMutexFree,
342 pthreadMutexEnter,
343 pthreadMutexTry,
344 pthreadMutexLeave,
345 #ifdef SQLITE_DEBUG
346 pthreadMutexHeld,
347 pthreadMutexNotheld
348 #else
351 #endif
354 return &sMutex;
357 #endif /* SQLITE_MUTEX_PTHREADS */