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.
10: **
11: *************************************************************************
12: ** This file contains the C functions that implement mutexes for pthreads
13: */
14: #include "sqliteInt.h"
15:
16: /*
17: ** The code in this file is only used if we are compiling threadsafe
18: ** under unix with pthreads.
19: **
20: ** Note that this implementation requires a version of pthreads that
21: ** supports recursive mutexes.
22: */
23: #ifdef SQLITE_MUTEX_PTHREADS
24:
25: #include <pthread.h>
26:
27: /*
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.
31: */
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
37:
38: /*
39: ** Each recursive mutex is an instance of the following structure.
40: */
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
49: };
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
55:
56: /*
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.
66: **
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.
71: */
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()));
75: }
76: static int pthreadMutexNotheld(sqlite3_mutex *p){
77: return p->nRef==0 || pthread_equal(p->owner, pthread_self())==0;
78: }
79: #endif
80:
81: /*
82: ** Initialize and deinitialize the mutex subsystem.
83: */
84: static int pthreadMutexInit(void){ return SQLITE_OK; }
85: static int pthreadMutexEnd(void){ return SQLITE_OK; }
86:
87: /*
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:
93: **
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_MEM2
100: ** <li> SQLITE_MUTEX_STATIC_PRNG
101: ** <li> SQLITE_MUTEX_STATIC_LRU
102: ** <li> SQLITE_MUTEX_STATIC_PMEM
103: ** </ul>
104: **
105: ** The first two constants cause sqlite3_mutex_alloc() to create
106: ** a new mutex. The new mutex is recursive when SQLITE_MUTEX_RECURSIVE
107: ** is used but not necessarily so when SQLITE_MUTEX_FAST is used.
108: ** The mutex implementation does not need to make a distinction
109: ** between SQLITE_MUTEX_RECURSIVE and SQLITE_MUTEX_FAST if it does
110: ** not want to. But SQLite will only request a recursive mutex in
111: ** cases where it really needs one. If a faster non-recursive mutex
112: ** implementation is available on the host platform, the mutex subsystem
113: ** might return such a mutex in response to SQLITE_MUTEX_FAST.
114: **
115: ** The other allowed parameters to sqlite3_mutex_alloc() each return
116: ** a pointer to a static preexisting mutex. Six static mutexes are
117: ** used by the current version of SQLite. Future versions of SQLite
118: ** may add additional static mutexes. Static mutexes are for internal
119: ** use by SQLite only. Applications that use SQLite mutexes should
120: ** use only the dynamic mutexes returned by SQLITE_MUTEX_FAST or
121: ** SQLITE_MUTEX_RECURSIVE.
122: **
123: ** Note that if one of the dynamic mutex parameters (SQLITE_MUTEX_FAST
124: ** or SQLITE_MUTEX_RECURSIVE) is used then sqlite3_mutex_alloc()
125: ** returns a different mutex on every call. But for the static
126: ** mutex types, the same mutex is returned on every call that has
127: ** the same type number.
128: */
129: static sqlite3_mutex *pthreadMutexAlloc(int iType){
130: static sqlite3_mutex staticMutexes[] = {
131: SQLITE3_MUTEX_INITIALIZER,
132: SQLITE3_MUTEX_INITIALIZER,
133: SQLITE3_MUTEX_INITIALIZER,
134: SQLITE3_MUTEX_INITIALIZER,
135: SQLITE3_MUTEX_INITIALIZER,
136: SQLITE3_MUTEX_INITIALIZER
137: };
138: sqlite3_mutex *p;
139: switch( iType ){
140: case SQLITE_MUTEX_RECURSIVE: {
141: p = sqlite3MallocZero( sizeof(*p) );
142: if( p ){
143: #ifdef SQLITE_HOMEGROWN_RECURSIVE_MUTEX
144: /* If recursive mutexes are not available, we will have to
145: ** build our own. See below. */
146: pthread_mutex_init(&p->mutex, 0);
147: #else
148: /* Use a recursive mutex if it is available */
149: pthread_mutexattr_t recursiveAttr;
150: pthread_mutexattr_init(&recursiveAttr);
151: pthread_mutexattr_settype(&recursiveAttr, PTHREAD_MUTEX_RECURSIVE);
152: pthread_mutex_init(&p->mutex, &recursiveAttr);
153: pthread_mutexattr_destroy(&recursiveAttr);
154: #endif
155: #if SQLITE_MUTEX_NREF
156: p->id = iType;
157: #endif
158: }
159: break;
160: }
161: case SQLITE_MUTEX_FAST: {
162: p = sqlite3MallocZero( sizeof(*p) );
163: if( p ){
164: #if SQLITE_MUTEX_NREF
165: p->id = iType;
166: #endif
167: pthread_mutex_init(&p->mutex, 0);
168: }
169: break;
170: }
171: default: {
172: assert( iType-2 >= 0 );
173: assert( iType-2 < ArraySize(staticMutexes) );
174: p = &staticMutexes[iType-2];
175: #if SQLITE_MUTEX_NREF
176: p->id = iType;
177: #endif
178: break;
179: }
180: }
181: return p;
182: }
183:
184:
185: /*
186: ** This routine deallocates a previously
187: ** allocated mutex. SQLite is careful to deallocate every
188: ** mutex that it allocates.
189: */
190: static void pthreadMutexFree(sqlite3_mutex *p){
191: assert( p->nRef==0 );
192: assert( p->id==SQLITE_MUTEX_FAST || p->id==SQLITE_MUTEX_RECURSIVE );
193: pthread_mutex_destroy(&p->mutex);
194: sqlite3_free(p);
195: }
196:
197: /*
198: ** The sqlite3_mutex_enter() and sqlite3_mutex_try() routines attempt
199: ** to enter a mutex. If another thread is already within the mutex,
200: ** sqlite3_mutex_enter() will block and sqlite3_mutex_try() will return
201: ** SQLITE_BUSY. The sqlite3_mutex_try() interface returns SQLITE_OK
202: ** upon successful entry. Mutexes created using SQLITE_MUTEX_RECURSIVE can
203: ** be entered multiple times by the same thread. In such cases the,
204: ** mutex must be exited an equal number of times before another thread
205: ** can enter. If the same thread tries to enter any other kind of mutex
206: ** more than once, the behavior is undefined.
207: */
208: static void pthreadMutexEnter(sqlite3_mutex *p){
209: assert( p->id==SQLITE_MUTEX_RECURSIVE || pthreadMutexNotheld(p) );
210:
211: #ifdef SQLITE_HOMEGROWN_RECURSIVE_MUTEX
212: /* If recursive mutexes are not available, then we have to grow
213: ** our own. This implementation assumes that pthread_equal()
214: ** is atomic - that it cannot be deceived into thinking self
215: ** and p->owner are equal if p->owner changes between two values
216: ** that are not equal to self while the comparison is taking place.
217: ** This implementation also assumes a coherent cache - that
218: ** separate processes cannot read different values from the same
219: ** address at the same time. If either of these two conditions
220: ** are not met, then the mutexes will fail and problems will result.
221: */
222: {
223: pthread_t self = pthread_self();
224: if( p->nRef>0 && pthread_equal(p->owner, self) ){
225: p->nRef++;
226: }else{
227: pthread_mutex_lock(&p->mutex);
228: assert( p->nRef==0 );
229: p->owner = self;
230: p->nRef = 1;
231: }
232: }
233: #else
234: /* Use the built-in recursive mutexes if they are available.
235: */
236: pthread_mutex_lock(&p->mutex);
237: #if SQLITE_MUTEX_NREF
238: assert( p->nRef>0 || p->owner==0 );
239: p->owner = pthread_self();
240: p->nRef++;
241: #endif
242: #endif
243:
244: #ifdef SQLITE_DEBUG
245: if( p->trace ){
246: printf("enter mutex %p (%d) with nRef=%d\n", p, p->trace, p->nRef);
247: }
248: #endif
249: }
250: static int pthreadMutexTry(sqlite3_mutex *p){
251: int rc;
252: assert( p->id==SQLITE_MUTEX_RECURSIVE || pthreadMutexNotheld(p) );
253:
254: #ifdef SQLITE_HOMEGROWN_RECURSIVE_MUTEX
255: /* If recursive mutexes are not available, then we have to grow
256: ** our own. This implementation assumes that pthread_equal()
257: ** is atomic - that it cannot be deceived into thinking self
258: ** and p->owner are equal if p->owner changes between two values
259: ** that are not equal to self while the comparison is taking place.
260: ** This implementation also assumes a coherent cache - that
261: ** separate processes cannot read different values from the same
262: ** address at the same time. If either of these two conditions
263: ** are not met, then the mutexes will fail and problems will result.
264: */
265: {
266: pthread_t self = pthread_self();
267: if( p->nRef>0 && pthread_equal(p->owner, self) ){
268: p->nRef++;
269: rc = SQLITE_OK;
270: }else if( pthread_mutex_trylock(&p->mutex)==0 ){
271: assert( p->nRef==0 );
272: p->owner = self;
273: p->nRef = 1;
274: rc = SQLITE_OK;
275: }else{
276: rc = SQLITE_BUSY;
277: }
278: }
279: #else
280: /* Use the built-in recursive mutexes if they are available.
281: */
282: if( pthread_mutex_trylock(&p->mutex)==0 ){
283: #if SQLITE_MUTEX_NREF
284: p->owner = pthread_self();
285: p->nRef++;
286: #endif
287: rc = SQLITE_OK;
288: }else{
289: rc = SQLITE_BUSY;
290: }
291: #endif
292:
293: #ifdef SQLITE_DEBUG
294: if( rc==SQLITE_OK && p->trace ){
295: printf("enter mutex %p (%d) with nRef=%d\n", p, p->trace, p->nRef);
296: }
297: #endif
298: return rc;
299: }
300:
301: /*
302: ** The sqlite3_mutex_leave() routine exits a mutex that was
303: ** previously entered by the same thread. The behavior
304: ** is undefined if the mutex is not currently entered or
305: ** is not currently allocated. SQLite will never do either.
306: */
307: static void pthreadMutexLeave(sqlite3_mutex *p){
308: assert( pthreadMutexHeld(p) );
309: #if SQLITE_MUTEX_NREF
310: p->nRef--;
311: if( p->nRef==0 ) p->owner = 0;
312: #endif
313: assert( p->nRef==0 || p->id==SQLITE_MUTEX_RECURSIVE );
314:
315: #ifdef SQLITE_HOMEGROWN_RECURSIVE_MUTEX
316: if( p->nRef==0 ){
317: pthread_mutex_unlock(&p->mutex);
318: }
319: #else
320: pthread_mutex_unlock(&p->mutex);
321: #endif
322:
323: #ifdef SQLITE_DEBUG
324: if( p->trace ){
325: printf("leave mutex %p (%d) with nRef=%d\n", p, p->trace, p->nRef);
326: }
327: #endif
328: }
329:
330: sqlite3_mutex_methods const *sqlite3DefaultMutex(void){
331: static const sqlite3_mutex_methods sMutex = {
332: pthreadMutexInit,
333: pthreadMutexEnd,
334: pthreadMutexAlloc,
335: pthreadMutexFree,
336: pthreadMutexEnter,
337: pthreadMutexTry,
338: pthreadMutexLeave,
339: #ifdef SQLITE_DEBUG
340: pthreadMutexHeld,
341: pthreadMutexNotheld
342: #else
343: 0,
344: 0
345: #endif
346: };
347:
348: return &sMutex;
349: }
350:
351: #endif /* SQLITE_MUTEX_PTHREADS */
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