src/mutex_unix.c

   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 */