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Diffstat (limited to '')
| -rw-r--r-- | src/btmutex.c | 309 |
1 files changed, 309 insertions, 0 deletions
diff --git a/src/btmutex.c b/src/btmutex.c new file mode 100644 index 0000000..45745b0 --- /dev/null +++ b/src/btmutex.c @@ -0,0 +1,309 @@ +/* +** 2007 August 27 +** +** The author disclaims copyright to this source code. In place of +** a legal notice, here is a blessing: +** +** May you do good and not evil. +** May you find forgiveness for yourself and forgive others. +** May you share freely, never taking more than you give. +** +************************************************************************* +** +** This file contains code used to implement mutexes on Btree objects. +** This code really belongs in btree.c. But btree.c is getting too +** big and we want to break it down some. This packaged seemed like +** a good breakout. +*/ +#include "btreeInt.h" +#ifndef SQLITE_OMIT_SHARED_CACHE +#if SQLITE_THREADSAFE + +/* +** Obtain the BtShared mutex associated with B-Tree handle p. Also, +** set BtShared.db to the database handle associated with p and the +** p->locked boolean to true. +*/ +static void lockBtreeMutex(Btree *p){ + assert( p->locked==0 ); + assert( sqlite3_mutex_notheld(p->pBt->mutex) ); + assert( sqlite3_mutex_held(p->db->mutex) ); + + sqlite3_mutex_enter(p->pBt->mutex); + p->pBt->db = p->db; + p->locked = 1; +} + +/* +** Release the BtShared mutex associated with B-Tree handle p and +** clear the p->locked boolean. +*/ +static void SQLITE_NOINLINE unlockBtreeMutex(Btree *p){ + BtShared *pBt = p->pBt; + assert( p->locked==1 ); + assert( sqlite3_mutex_held(pBt->mutex) ); + assert( sqlite3_mutex_held(p->db->mutex) ); + assert( p->db==pBt->db ); + + sqlite3_mutex_leave(pBt->mutex); + p->locked = 0; +} + +/* Forward reference */ +static void SQLITE_NOINLINE btreeLockCarefully(Btree *p); + +/* +** Enter a mutex on the given BTree object. +** +** If the object is not sharable, then no mutex is ever required +** and this routine is a no-op. The underlying mutex is non-recursive. +** But we keep a reference count in Btree.wantToLock so the behavior +** of this interface is recursive. +** +** To avoid deadlocks, multiple Btrees are locked in the same order +** by all database connections. The p->pNext is a list of other +** Btrees belonging to the same database connection as the p Btree +** which need to be locked after p. If we cannot get a lock on +** p, then first unlock all of the others on p->pNext, then wait +** for the lock to become available on p, then relock all of the +** subsequent Btrees that desire a lock. +*/ +void sqlite3BtreeEnter(Btree *p){ + /* Some basic sanity checking on the Btree. The list of Btrees + ** connected by pNext and pPrev should be in sorted order by + ** Btree.pBt value. All elements of the list should belong to + ** the same connection. Only shared Btrees are on the list. */ + assert( p->pNext==0 || p->pNext->pBt>p->pBt ); + assert( p->pPrev==0 || p->pPrev->pBt<p->pBt ); + assert( p->pNext==0 || p->pNext->db==p->db ); + assert( p->pPrev==0 || p->pPrev->db==p->db ); + assert( p->sharable || (p->pNext==0 && p->pPrev==0) ); + + /* Check for locking consistency */ + assert( !p->locked || p->wantToLock>0 ); + assert( p->sharable || p->wantToLock==0 ); + + /* We should already hold a lock on the database connection */ + assert( sqlite3_mutex_held(p->db->mutex) ); + + /* Unless the database is sharable and unlocked, then BtShared.db + ** should already be set correctly. */ + assert( (p->locked==0 && p->sharable) || p->pBt->db==p->db ); + + if( !p->sharable ) return; + p->wantToLock++; + if( p->locked ) return; + btreeLockCarefully(p); +} + +/* This is a helper function for sqlite3BtreeLock(). By moving +** complex, but seldom used logic, out of sqlite3BtreeLock() and +** into this routine, we avoid unnecessary stack pointer changes +** and thus help the sqlite3BtreeLock() routine to run much faster +** in the common case. +*/ +static void SQLITE_NOINLINE btreeLockCarefully(Btree *p){ + Btree *pLater; + + /* In most cases, we should be able to acquire the lock we + ** want without having to go through the ascending lock + ** procedure that follows. Just be sure not to block. + */ + if( sqlite3_mutex_try(p->pBt->mutex)==SQLITE_OK ){ + p->pBt->db = p->db; + p->locked = 1; + return; + } + + /* To avoid deadlock, first release all locks with a larger + ** BtShared address. Then acquire our lock. Then reacquire + ** the other BtShared locks that we used to hold in ascending + ** order. + */ + for(pLater=p->pNext; pLater; pLater=pLater->pNext){ + assert( pLater->sharable ); + assert( pLater->pNext==0 || pLater->pNext->pBt>pLater->pBt ); + assert( !pLater->locked || pLater->wantToLock>0 ); + if( pLater->locked ){ + unlockBtreeMutex(pLater); + } + } + lockBtreeMutex(p); + for(pLater=p->pNext; pLater; pLater=pLater->pNext){ + if( pLater->wantToLock ){ + lockBtreeMutex(pLater); + } + } +} + + +/* +** Exit the recursive mutex on a Btree. +*/ +void sqlite3BtreeLeave(Btree *p){ + assert( sqlite3_mutex_held(p->db->mutex) ); + if( p->sharable ){ + assert( p->wantToLock>0 ); + p->wantToLock--; + if( p->wantToLock==0 ){ + unlockBtreeMutex(p); + } + } +} + +#ifndef NDEBUG +/* +** Return true if the BtShared mutex is held on the btree, or if the +** B-Tree is not marked as sharable. +** +** This routine is used only from within assert() statements. +*/ +int sqlite3BtreeHoldsMutex(Btree *p){ + assert( p->sharable==0 || p->locked==0 || p->wantToLock>0 ); + assert( p->sharable==0 || p->locked==0 || p->db==p->pBt->db ); + assert( p->sharable==0 || p->locked==0 || sqlite3_mutex_held(p->pBt->mutex) ); + assert( p->sharable==0 || p->locked==0 || sqlite3_mutex_held(p->db->mutex) ); + + return (p->sharable==0 || p->locked); +} +#endif + + +/* +** Enter the mutex on every Btree associated with a database +** connection. This is needed (for example) prior to parsing +** a statement since we will be comparing table and column names +** against all schemas and we do not want those schemas being +** reset out from under us. +** +** There is a corresponding leave-all procedures. +** +** Enter the mutexes in accending order by BtShared pointer address +** to avoid the possibility of deadlock when two threads with +** two or more btrees in common both try to lock all their btrees +** at the same instant. +*/ +static void SQLITE_NOINLINE btreeEnterAll(sqlite3 *db){ + int i; + int skipOk = 1; + Btree *p; + assert( sqlite3_mutex_held(db->mutex) ); + for(i=0; i<db->nDb; i++){ + p = db->aDb[i].pBt; + if( p && p->sharable ){ + sqlite3BtreeEnter(p); + skipOk = 0; + } + } + db->noSharedCache = skipOk; +} +void sqlite3BtreeEnterAll(sqlite3 *db){ + if( db->noSharedCache==0 ) btreeEnterAll(db); +} +static void SQLITE_NOINLINE btreeLeaveAll(sqlite3 *db){ + int i; + Btree *p; + assert( sqlite3_mutex_held(db->mutex) ); + for(i=0; i<db->nDb; i++){ + p = db->aDb[i].pBt; + if( p ) sqlite3BtreeLeave(p); + } +} +void sqlite3BtreeLeaveAll(sqlite3 *db){ + if( db->noSharedCache==0 ) btreeLeaveAll(db); +} + +#ifndef NDEBUG +/* +** Return true if the current thread holds the database connection +** mutex and all required BtShared mutexes. +** +** This routine is used inside assert() statements only. +*/ +int sqlite3BtreeHoldsAllMutexes(sqlite3 *db){ + int i; + if( !sqlite3_mutex_held(db->mutex) ){ + return 0; + } + for(i=0; i<db->nDb; i++){ + Btree *p; + p = db->aDb[i].pBt; + if( p && p->sharable && + (p->wantToLock==0 || !sqlite3_mutex_held(p->pBt->mutex)) ){ + return 0; + } + } + return 1; +} +#endif /* NDEBUG */ + +#ifndef NDEBUG +/* +** Return true if the correct mutexes are held for accessing the +** db->aDb[iDb].pSchema structure. The mutexes required for schema +** access are: +** +** (1) The mutex on db +** (2) if iDb!=1, then the mutex on db->aDb[iDb].pBt. +** +** If pSchema is not NULL, then iDb is computed from pSchema and +** db using sqlite3SchemaToIndex(). +*/ +int sqlite3SchemaMutexHeld(sqlite3 *db, int iDb, Schema *pSchema){ + Btree *p; + assert( db!=0 ); + if( db->pVfs==0 && db->nDb==0 ) return 1; + if( pSchema ) iDb = sqlite3SchemaToIndex(db, pSchema); + assert( iDb>=0 && iDb<db->nDb ); + if( !sqlite3_mutex_held(db->mutex) ) return 0; + if( iDb==1 ) return 1; + p = db->aDb[iDb].pBt; + assert( p!=0 ); + return p->sharable==0 || p->locked==1; +} +#endif /* NDEBUG */ + +#else /* SQLITE_THREADSAFE>0 above. SQLITE_THREADSAFE==0 below */ +/* +** The following are special cases for mutex enter routines for use +** in single threaded applications that use shared cache. Except for +** these two routines, all mutex operations are no-ops in that case and +** are null #defines in btree.h. +** +** If shared cache is disabled, then all btree mutex routines, including +** the ones below, are no-ops and are null #defines in btree.h. +*/ + +void sqlite3BtreeEnter(Btree *p){ + p->pBt->db = p->db; +} +void sqlite3BtreeEnterAll(sqlite3 *db){ + int i; + for(i=0; i<db->nDb; i++){ + Btree *p = db->aDb[i].pBt; + if( p ){ + p->pBt->db = p->db; + } + } +} +#endif /* if SQLITE_THREADSAFE */ + +#ifndef SQLITE_OMIT_INCRBLOB +/* +** Enter a mutex on a Btree given a cursor owned by that Btree. +** +** These entry points are used by incremental I/O only. Enter() is required +** any time OMIT_SHARED_CACHE is not defined, regardless of whether or not +** the build is threadsafe. Leave() is only required by threadsafe builds. +*/ +void sqlite3BtreeEnterCursor(BtCursor *pCur){ + sqlite3BtreeEnter(pCur->pBtree); +} +# if SQLITE_THREADSAFE +void sqlite3BtreeLeaveCursor(BtCursor *pCur){ + sqlite3BtreeLeave(pCur->pBtree); +} +# endif +#endif /* ifndef SQLITE_OMIT_INCRBLOB */ + +#endif /* ifndef SQLITE_OMIT_SHARED_CACHE */ |