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Copy pathbtmutex.c
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308 lines (283 loc) · 9.1 KB
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/*
** 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->skipBtreeMutex = skipOk;
}
void sqlite3BtreeEnterAll(sqlite3 *db){
if( db->skipBtreeMutex==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->skipBtreeMutex==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( 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 */