src/btmutex.c

   1 /*
   2 ** 2007 August 27
   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 **
  13 ** This file contains code used to implement mutexes on Btree objects.
  14 ** This code really belongs in btree.c.  But btree.c is getting too
  15 ** big and we want to break it down some.  This packaged seemed like
  16 ** a good breakout.
  17 */
  18 #include "btreeInt.h"
  19 #ifndef SQLITE_OMIT_SHARED_CACHE
  20 #if SQLITE_THREADSAFE
  21
  22 /*
  23 ** Obtain the BtShared mutex associated with B-Tree handle p. Also,
  24 ** set BtShared.db to the database handle associated with p and the
  25 ** p->locked boolean to true.
  26 */
  27 static void lockBtreeMutex(Btree *p){
  28   assert( p->locked==0 );
  29   assert( sqlite3_mutex_notheld(p->pBt->mutex) );
  30   assert( sqlite3_mutex_held(p->db->mutex) );
  31
  32   sqlite3_mutex_enter(p->pBt->mutex);
  33   p->pBt->db = p->db;
  34   p->locked = 1;
  35 }
  36
  37 /*
  38 ** Release the BtShared mutex associated with B-Tree handle p and
  39 ** clear the p->locked boolean.
  40 */
  41 static void unlockBtreeMutex(Btree *p){
  42   BtShared *pBt = p->pBt;
  43   assert( p->locked==1 );
  44   assert( sqlite3_mutex_held(pBt->mutex) );
  45   assert( sqlite3_mutex_held(p->db->mutex) );
  46   assert( p->db==pBt->db );
  47
  48   sqlite3_mutex_leave(pBt->mutex);
  49   p->locked = 0;
  50 }
  51
  52 /*
  53 ** Enter a mutex on the given BTree object.
  54 **
  55 ** If the object is not sharable, then no mutex is ever required
  56 ** and this routine is a no-op.  The underlying mutex is non-recursive.
  57 ** But we keep a reference count in Btree.wantToLock so the behavior
  58 ** of this interface is recursive.
  59 **
  60 ** To avoid deadlocks, multiple Btrees are locked in the same order
  61 ** by all database connections.  The p->pNext is a list of other
  62 ** Btrees belonging to the same database connection as the p Btree
  63 ** which need to be locked after p.  If we cannot get a lock on
  64 ** p, then first unlock all of the others on p->pNext, then wait
  65 ** for the lock to become available on p, then relock all of the
  66 ** subsequent Btrees that desire a lock.
  67 */
  68 void sqlite3BtreeEnter(Btree *p){
  69   Btree *pLater;
  70
  71   /* Some basic sanity checking on the Btree.  The list of Btrees
  72   ** connected by pNext and pPrev should be in sorted order by
  73   ** Btree.pBt value. All elements of the list should belong to
  74   ** the same connection. Only shared Btrees are on the list. */
  75   assert( p->pNext==0 || p->pNext->pBt>p->pBt );
  76   assert( p->pPrev==0 || p->pPrev->pBt<p->pBt );
  77   assert( p->pNext==0 || p->pNext->db==p->db );
  78   assert( p->pPrev==0 || p->pPrev->db==p->db );
  79   assert( p->sharable || (p->pNext==0 && p->pPrev==0) );
  80
  81   /* Check for locking consistency */
  82   assert( !p->locked || p->wantToLock>0 );
  83   assert( p->sharable || p->wantToLock==0 );
  84
  85   /* We should already hold a lock on the database connection */
  86   assert( sqlite3_mutex_held(p->db->mutex) );
  87
  88   /* Unless the database is sharable and unlocked, then BtShared.db
  89   ** should already be set correctly. */
  90   assert( (p->locked==0 && p->sharable) || p->pBt->db==p->db );
  91
  92   if( !p->sharable ) return;
  93   p->wantToLock++;
  94   if( p->locked ) return;
  95
  96   /* In most cases, we should be able to acquire the lock we
  97   ** want without having to go throught the ascending lock
  98   ** procedure that follows.  Just be sure not to block.
  99   */
 100   if( sqlite3_mutex_try(p->pBt->mutex)==SQLITE_OK ){
 101     p->pBt->db = p->db;
 102     p->locked = 1;
 103     return;
 104   }
 105
 106   /* To avoid deadlock, first release all locks with a larger
 107   ** BtShared address.  Then acquire our lock.  Then reacquire
 108   ** the other BtShared locks that we used to hold in ascending
 109   ** order.
 110   */
 111   for(pLater=p->pNext; pLater; pLater=pLater->pNext){
 112     assert( pLater->sharable );
 113     assert( pLater->pNext==0 || pLater->pNext->pBt>pLater->pBt );
 114     assert( !pLater->locked || pLater->wantToLock>0 );
 115     if( pLater->locked ){
 116       unlockBtreeMutex(pLater);
 117     }
 118   }
 119   lockBtreeMutex(p);
 120   for(pLater=p->pNext; pLater; pLater=pLater->pNext){
 121     if( pLater->wantToLock ){
 122       lockBtreeMutex(pLater);
 123     }
 124   }
 125 }
 126
 127 /*
 128 ** Exit the recursive mutex on a Btree.
 129 */
 130 void sqlite3BtreeLeave(Btree *p){
 131   if( p->sharable ){
 132     assert( p->wantToLock>0 );
 133     p->wantToLock--;
 134     if( p->wantToLock==0 ){
 135       unlockBtreeMutex(p);
 136     }
 137   }
 138 }
 139
 140 #ifndef NDEBUG
 141 /*
 142 ** Return true if the BtShared mutex is held on the btree, or if the
 143 ** B-Tree is not marked as sharable.
 144 **
 145 ** This routine is used only from within assert() statements.
 146 */
 147 int sqlite3BtreeHoldsMutex(Btree *p){
 148   assert( p->sharable==0 || p->locked==0 || p->wantToLock>0 );
 149   assert( p->sharable==0 || p->locked==0 || p->db==p->pBt->db );
 150   assert( p->sharable==0 || p->locked==0 || sqlite3_mutex_held(p->pBt->mutex) );
 151   assert( p->sharable==0 || p->locked==0 || sqlite3_mutex_held(p->db->mutex) );
 152
 153   return (p->sharable==0 || p->locked);
 154 }
 155 #endif
 156
 157
 158 #ifndef SQLITE_OMIT_INCRBLOB
 159 /*
 160 ** Enter and leave a mutex on a Btree given a cursor owned by that
 161 ** Btree.  These entry points are used by incremental I/O and can be
 162 ** omitted if that module is not used.
 163 */
 164 void sqlite3BtreeEnterCursor(BtCursor *pCur){
 165   sqlite3BtreeEnter(pCur->pBtree);
 166 }
 167 void sqlite3BtreeLeaveCursor(BtCursor *pCur){
 168   sqlite3BtreeLeave(pCur->pBtree);
 169 }
 170 #endif /* SQLITE_OMIT_INCRBLOB */
 171
 172
 173 /*
 174 ** Enter the mutex on every Btree associated with a database
 175 ** connection.  This is needed (for example) prior to parsing
 176 ** a statement since we will be comparing table and column names
 177 ** against all schemas and we do not want those schemas being
 178 ** reset out from under us.
 179 **
 180 ** There is a corresponding leave-all procedures.
 181 **
 182 ** Enter the mutexes in accending order by BtShared pointer address
 183 ** to avoid the possibility of deadlock when two threads with
 184 ** two or more btrees in common both try to lock all their btrees
 185 ** at the same instant.
 186 */
 187 void sqlite3BtreeEnterAll(sqlite3 *db){
 188   int i;
 189   Btree *p;
 190   assert( sqlite3_mutex_held(db->mutex) );
 191   for(i=0; i<db->nDb; i++){
 192     p = db->aDb[i].pBt;
 193     if( p ) sqlite3BtreeEnter(p);
 194   }
 195 }
 196 void sqlite3BtreeLeaveAll(sqlite3 *db){
 197   int i;
 198   Btree *p;
 199   assert( sqlite3_mutex_held(db->mutex) );
 200   for(i=0; i<db->nDb; i++){
 201     p = db->aDb[i].pBt;
 202     if( p ) sqlite3BtreeLeave(p);
 203   }
 204 }
 205
 206 /*
 207 ** Return true if a particular Btree requires a lock.  Return FALSE if
 208 ** no lock is ever required since it is not sharable.
 209 */
 210 int sqlite3BtreeSharable(Btree *p){
 211   return p->sharable;
 212 }
 213
 214 #ifndef NDEBUG
 215 /*
 216 ** Return true if the current thread holds the database connection
 217 ** mutex and all required BtShared mutexes.
 218 **
 219 ** This routine is used inside assert() statements only.
 220 */
 221 int sqlite3BtreeHoldsAllMutexes(sqlite3 *db){
 222   int i;
 223   if( !sqlite3_mutex_held(db->mutex) ){
 224     return 0;
 225   }
 226   for(i=0; i<db->nDb; i++){
 227     Btree *p;
 228     p = db->aDb[i].pBt;
 229     if( p && p->sharable &&
 230          (p->wantToLock==0 || !sqlite3_mutex_held(p->pBt->mutex)) ){
 231       return 0;
 232     }
 233   }
 234   return 1;
 235 }
 236 #endif /* NDEBUG */
 237
 238 #ifndef NDEBUG
 239 /*
 240 ** Return true if the correct mutexes are held for accessing the
 241 ** db->aDb[iDb].pSchema structure.  The mutexes required for schema
 242 ** access are:
 243 **
 244 **   (1) The mutex on db
 245 **   (2) if iDb!=1, then the mutex on db->aDb[iDb].pBt.
 246 **
 247 ** If pSchema is not NULL, then iDb is computed from pSchema and
 248 ** db using sqlite3SchemaToIndex().
 249 */
 250 int sqlite3SchemaMutexHeld(sqlite3 *db, int iDb, Schema *pSchema){
 251   Btree *p;
 252   assert( db!=0 );
 253   if( pSchema ) iDb = sqlite3SchemaToIndex(db, pSchema);
 254   assert( iDb>=0 && iDb<db->nDb );
 255   if( !sqlite3_mutex_held(db->mutex) ) return 0;
 256   if( iDb==1 ) return 1;
 257   p = db->aDb[iDb].pBt;
 258   assert( p!=0 );
 259   return p->sharable==0 || p->locked==1;
 260 }
 261 #endif /* NDEBUG */
 262
 263 #else /* SQLITE_THREADSAFE>0 above.  SQLITE_THREADSAFE==0 below */
 264 /*
 265 ** The following are special cases for mutex enter routines for use
 266 ** in single threaded applications that use shared cache.  Except for
 267 ** these two routines, all mutex operations are no-ops in that case and
 268 ** are null #defines in btree.h.
 269 **
 270 ** If shared cache is disabled, then all btree mutex routines, including
 271 ** the ones below, are no-ops and are null #defines in btree.h.
 272 */
 273
 274 void sqlite3BtreeEnter(Btree *p){
 275   p->pBt->db = p->db;
 276 }
 277 void sqlite3BtreeEnterAll(sqlite3 *db){
 278   int i;
 279   for(i=0; i<db->nDb; i++){
 280     Btree *p = db->aDb[i].pBt;
 281     if( p ){
 282       p->pBt->db = p->db;
 283     }
 284   }
 285 }
 286 #endif /* if SQLITE_THREADSAFE */
 287 #endif /* ifndef SQLITE_OMIT_SHARED_CACHE */