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351 lines (331 loc) · 11.1 KB
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/*
** 2007 August 28
**
** 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 the C functions that implement mutexes for pthreads
*/
#include "sqliteInt.h"
/*
** The code in this file is only used if we are compiling threadsafe
** under unix with pthreads.
**
** Note that this implementation requires a version of pthreads that
** supports recursive mutexes.
*/
#ifdef SQLITE_MUTEX_PTHREADS
#include <pthread.h>
/*
** The sqlite3_mutex.id, sqlite3_mutex.nRef, and sqlite3_mutex.owner fields
** are necessary under two condidtions: (1) Debug builds and (2) using
** home-grown mutexes. Encapsulate these conditions into a single #define.
*/
#if defined(SQLITE_DEBUG) || defined(SQLITE_HOMEGROWN_RECURSIVE_MUTEX)
# define SQLITE_MUTEX_NREF 1
#else
# define SQLITE_MUTEX_NREF 0
#endif
/*
** Each recursive mutex is an instance of the following structure.
*/
struct sqlite3_mutex {
pthread_mutex_t mutex; /* Mutex controlling the lock */
#if SQLITE_MUTEX_NREF
int id; /* Mutex type */
volatile int nRef; /* Number of entrances */
volatile pthread_t owner; /* Thread that is within this mutex */
int trace; /* True to trace changes */
#endif
};
#if SQLITE_MUTEX_NREF
#define SQLITE3_MUTEX_INITIALIZER { PTHREAD_MUTEX_INITIALIZER, 0, 0, (pthread_t)0, 0 }
#else
#define SQLITE3_MUTEX_INITIALIZER { PTHREAD_MUTEX_INITIALIZER }
#endif
/*
** The sqlite3_mutex_held() and sqlite3_mutex_notheld() routine are
** intended for use only inside assert() statements. On some platforms,
** there might be race conditions that can cause these routines to
** deliver incorrect results. In particular, if pthread_equal() is
** not an atomic operation, then these routines might delivery
** incorrect results. On most platforms, pthread_equal() is a
** comparison of two integers and is therefore atomic. But we are
** told that HPUX is not such a platform. If so, then these routines
** will not always work correctly on HPUX.
**
** On those platforms where pthread_equal() is not atomic, SQLite
** should be compiled without -DSQLITE_DEBUG and with -DNDEBUG to
** make sure no assert() statements are evaluated and hence these
** routines are never called.
*/
#if !defined(NDEBUG) || defined(SQLITE_DEBUG)
static int pthreadMutexHeld(sqlite3_mutex *p){
return (p->nRef!=0 && pthread_equal(p->owner, pthread_self()));
}
static int pthreadMutexNotheld(sqlite3_mutex *p){
return p->nRef==0 || pthread_equal(p->owner, pthread_self())==0;
}
#endif
/*
** Initialize and deinitialize the mutex subsystem.
*/
static int pthreadMutexInit(void){ return SQLITE_OK; }
static int pthreadMutexEnd(void){ return SQLITE_OK; }
/*
** The sqlite3_mutex_alloc() routine allocates a new
** mutex and returns a pointer to it. If it returns NULL
** that means that a mutex could not be allocated. SQLite
** will unwind its stack and return an error. The argument
** to sqlite3_mutex_alloc() is one of these integer constants:
**
** <ul>
** <li> SQLITE_MUTEX_FAST
** <li> SQLITE_MUTEX_RECURSIVE
** <li> SQLITE_MUTEX_STATIC_MASTER
** <li> SQLITE_MUTEX_STATIC_MEM
** <li> SQLITE_MUTEX_STATIC_MEM2
** <li> SQLITE_MUTEX_STATIC_PRNG
** <li> SQLITE_MUTEX_STATIC_LRU
** <li> SQLITE_MUTEX_STATIC_PMEM
** </ul>
**
** The first two constants cause sqlite3_mutex_alloc() to create
** a new mutex. The new mutex is recursive when SQLITE_MUTEX_RECURSIVE
** is used but not necessarily so when SQLITE_MUTEX_FAST is used.
** The mutex implementation does not need to make a distinction
** between SQLITE_MUTEX_RECURSIVE and SQLITE_MUTEX_FAST if it does
** not want to. But SQLite will only request a recursive mutex in
** cases where it really needs one. If a faster non-recursive mutex
** implementation is available on the host platform, the mutex subsystem
** might return such a mutex in response to SQLITE_MUTEX_FAST.
**
** The other allowed parameters to sqlite3_mutex_alloc() each return
** a pointer to a static preexisting mutex. Six static mutexes are
** used by the current version of SQLite. Future versions of SQLite
** may add additional static mutexes. Static mutexes are for internal
** use by SQLite only. Applications that use SQLite mutexes should
** use only the dynamic mutexes returned by SQLITE_MUTEX_FAST or
** SQLITE_MUTEX_RECURSIVE.
**
** Note that if one of the dynamic mutex parameters (SQLITE_MUTEX_FAST
** or SQLITE_MUTEX_RECURSIVE) is used then sqlite3_mutex_alloc()
** returns a different mutex on every call. But for the static
** mutex types, the same mutex is returned on every call that has
** the same type number.
*/
static sqlite3_mutex *pthreadMutexAlloc(int iType){
static sqlite3_mutex staticMutexes[] = {
SQLITE3_MUTEX_INITIALIZER,
SQLITE3_MUTEX_INITIALIZER,
SQLITE3_MUTEX_INITIALIZER,
SQLITE3_MUTEX_INITIALIZER,
SQLITE3_MUTEX_INITIALIZER,
SQLITE3_MUTEX_INITIALIZER
};
sqlite3_mutex *p;
switch( iType ){
case SQLITE_MUTEX_RECURSIVE: {
p = sqlite3MallocZero( sizeof(*p) );
if( p ){
#ifdef SQLITE_HOMEGROWN_RECURSIVE_MUTEX
/* If recursive mutexes are not available, we will have to
** build our own. See below. */
pthread_mutex_init(&p->mutex, 0);
#else
/* Use a recursive mutex if it is available */
pthread_mutexattr_t recursiveAttr;
pthread_mutexattr_init(&recursiveAttr);
pthread_mutexattr_settype(&recursiveAttr, PTHREAD_MUTEX_RECURSIVE);
pthread_mutex_init(&p->mutex, &recursiveAttr);
pthread_mutexattr_destroy(&recursiveAttr);
#endif
#if SQLITE_MUTEX_NREF
p->id = iType;
#endif
}
break;
}
case SQLITE_MUTEX_FAST: {
p = sqlite3MallocZero( sizeof(*p) );
if( p ){
#if SQLITE_MUTEX_NREF
p->id = iType;
#endif
pthread_mutex_init(&p->mutex, 0);
}
break;
}
default: {
assert( iType-2 >= 0 );
assert( iType-2 < ArraySize(staticMutexes) );
p = &staticMutexes[iType-2];
#if SQLITE_MUTEX_NREF
p->id = iType;
#endif
break;
}
}
return p;
}
/*
** This routine deallocates a previously
** allocated mutex. SQLite is careful to deallocate every
** mutex that it allocates.
*/
static void pthreadMutexFree(sqlite3_mutex *p){
assert( p->nRef==0 );
assert( p->id==SQLITE_MUTEX_FAST || p->id==SQLITE_MUTEX_RECURSIVE );
pthread_mutex_destroy(&p->mutex);
sqlite3_free(p);
}
/*
** The sqlite3_mutex_enter() and sqlite3_mutex_try() routines attempt
** to enter a mutex. If another thread is already within the mutex,
** sqlite3_mutex_enter() will block and sqlite3_mutex_try() will return
** SQLITE_BUSY. The sqlite3_mutex_try() interface returns SQLITE_OK
** upon successful entry. Mutexes created using SQLITE_MUTEX_RECURSIVE can
** be entered multiple times by the same thread. In such cases the,
** mutex must be exited an equal number of times before another thread
** can enter. If the same thread tries to enter any other kind of mutex
** more than once, the behavior is undefined.
*/
static void pthreadMutexEnter(sqlite3_mutex *p){
assert( p->id==SQLITE_MUTEX_RECURSIVE || pthreadMutexNotheld(p) );
#ifdef SQLITE_HOMEGROWN_RECURSIVE_MUTEX
/* If recursive mutexes are not available, then we have to grow
** our own. This implementation assumes that pthread_equal()
** is atomic - that it cannot be deceived into thinking self
** and p->owner are equal if p->owner changes between two values
** that are not equal to self while the comparison is taking place.
** This implementation also assumes a coherent cache - that
** separate processes cannot read different values from the same
** address at the same time. If either of these two conditions
** are not met, then the mutexes will fail and problems will result.
*/
{
pthread_t self = pthread_self();
if( p->nRef>0 && pthread_equal(p->owner, self) ){
p->nRef++;
}else{
pthread_mutex_lock(&p->mutex);
assert( p->nRef==0 );
p->owner = self;
p->nRef = 1;
}
}
#else
/* Use the built-in recursive mutexes if they are available.
*/
pthread_mutex_lock(&p->mutex);
#if SQLITE_MUTEX_NREF
assert( p->nRef>0 || p->owner==0 );
p->owner = pthread_self();
p->nRef++;
#endif
#endif
#ifdef SQLITE_DEBUG
if( p->trace ){
printf("enter mutex %p (%d) with nRef=%d\n", p, p->trace, p->nRef);
}
#endif
}
static int pthreadMutexTry(sqlite3_mutex *p){
int rc;
assert( p->id==SQLITE_MUTEX_RECURSIVE || pthreadMutexNotheld(p) );
#ifdef SQLITE_HOMEGROWN_RECURSIVE_MUTEX
/* If recursive mutexes are not available, then we have to grow
** our own. This implementation assumes that pthread_equal()
** is atomic - that it cannot be deceived into thinking self
** and p->owner are equal if p->owner changes between two values
** that are not equal to self while the comparison is taking place.
** This implementation also assumes a coherent cache - that
** separate processes cannot read different values from the same
** address at the same time. If either of these two conditions
** are not met, then the mutexes will fail and problems will result.
*/
{
pthread_t self = pthread_self();
if( p->nRef>0 && pthread_equal(p->owner, self) ){
p->nRef++;
rc = SQLITE_OK;
}else if( pthread_mutex_trylock(&p->mutex)==0 ){
assert( p->nRef==0 );
p->owner = self;
p->nRef = 1;
rc = SQLITE_OK;
}else{
rc = SQLITE_BUSY;
}
}
#else
/* Use the built-in recursive mutexes if they are available.
*/
if( pthread_mutex_trylock(&p->mutex)==0 ){
#if SQLITE_MUTEX_NREF
p->owner = pthread_self();
p->nRef++;
#endif
rc = SQLITE_OK;
}else{
rc = SQLITE_BUSY;
}
#endif
#ifdef SQLITE_DEBUG
if( rc==SQLITE_OK && p->trace ){
printf("enter mutex %p (%d) with nRef=%d\n", p, p->trace, p->nRef);
}
#endif
return rc;
}
/*
** The sqlite3_mutex_leave() routine exits a mutex that was
** previously entered by the same thread. The behavior
** is undefined if the mutex is not currently entered or
** is not currently allocated. SQLite will never do either.
*/
static void pthreadMutexLeave(sqlite3_mutex *p){
assert( pthreadMutexHeld(p) );
#if SQLITE_MUTEX_NREF
p->nRef--;
if( p->nRef==0 ) p->owner = 0;
#endif
assert( p->nRef==0 || p->id==SQLITE_MUTEX_RECURSIVE );
#ifdef SQLITE_HOMEGROWN_RECURSIVE_MUTEX
if( p->nRef==0 ){
pthread_mutex_unlock(&p->mutex);
}
#else
pthread_mutex_unlock(&p->mutex);
#endif
#ifdef SQLITE_DEBUG
if( p->trace ){
printf("leave mutex %p (%d) with nRef=%d\n", p, p->trace, p->nRef);
}
#endif
}
sqlite3_mutex_methods const *sqlite3DefaultMutex(void){
static const sqlite3_mutex_methods sMutex = {
pthreadMutexInit,
pthreadMutexEnd,
pthreadMutexAlloc,
pthreadMutexFree,
pthreadMutexEnter,
pthreadMutexTry,
pthreadMutexLeave,
#ifdef SQLITE_DEBUG
pthreadMutexHeld,
pthreadMutexNotheld
#else
0,
0
#endif
};
return &sMutex;
}
#endif /* SQLITE_MUTEX_PTHREADS */