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Clarify the conditions under which homegrown recursive mutexes work

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(they require a coherent cache) and only enable them if there is an
explicit #define so as to avoid accidental use on platforms that do
not meet the constraints.  Ticket #2805. (CVS 4575)

FossilOrigin-Name: 80299eebddba9aac4c1bc36ffa2b440bffbf1751
This commit is contained in:
drh
2007-11-28 13:55:55 +00:00
parent 4aa2bfe67d
commit 5f3d652394
3 changed files with 19 additions and 13 deletions
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18
src/mutex_unix.c
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@@ -11,7 +11,7 @@
*************************************************************************
** This file contains the C functions that implement mutexes for pthreads
**
** $Id: mutex_unix.c,v 1.3 2007/11/28 00:51:35 drh Exp $
** $Id: mutex_unix.c,v 1.4 2007/11/28 13:55:55 drh Exp $
*/
#include "sqliteInt.h"
@@ -94,7 +94,7 @@ sqlite3_mutex *sqlite3_mutex_alloc(int iType){
case SQLITE_MUTEX_RECURSIVE: {
p = sqlite3MallocZero( sizeof(*p) );
if( p ){
#ifdef PTHREAD_MUTEX_RECURSIVE
#ifndef SQLITE_HOMEGROWN_RECURSIVE_MUTEX
/* Use a recursive mutex if it is available */
pthread_mutexattr_t recursiveAttr;
pthread_mutexattr_init(&recursiveAttr);
@@ -158,9 +158,8 @@ void sqlite3_mutex_enter(sqlite3_mutex *p){
assert( p );
assert( p->id==SQLITE_MUTEX_RECURSIVE || sqlite3_mutex_notheld(p) );
#ifdef PTHREAD_MUTEX_RECURSIVE
#ifndef SQLITE_HOMEGROWN_RECURSIVE_MUTEX
/* Use the built-in recursive mutexes if they are available.
** That they are not available on all systems.
*/
pthread_mutex_lock(&p->mutex);
p->owner = pthread_self();
@@ -171,6 +170,10 @@ void sqlite3_mutex_enter(sqlite3_mutex *p){
** 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();
@@ -196,9 +199,8 @@ int sqlite3_mutex_try(sqlite3_mutex *p){
assert( p );
assert( p->id==SQLITE_MUTEX_RECURSIVE || sqlite3_mutex_notheld(p) );
#ifdef PTHREAD_MUTEX_RECURSIVE
#ifndef SQLITE_HOMEGROWN_RECURSIVE_MUTEX
/* Use the built-in recursive mutexes if they are available.
** That they are not available on all systems.
*/
if( pthread_mutex_trylock(&p->mutex)==0 ){
p->owner = pthread_self();
@@ -213,6 +215,10 @@ int sqlite3_mutex_try(sqlite3_mutex *p){
** 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();