#endif
#if !USE_MPROTECT_SNAPSHOT
+/** @statics
+* These variables are necessary because the stack is shared region and
+* there exists a race between all processes executing the same function.
+* To avoid the problem above, we require variables allocated in 'safe' regions.
+* The bug was actually observed with the forkID, these variables below are
+* used to indicate the various contexts to which to switch to.
+*
+* @savedSnapshotContext: contains the point to which takesnapshot() call should switch to.
+* @savedUserSnapshotContext: contains the point to which the process whose snapshotid is equal to the rollbackid should switch to
+* @snapshotid: it is a running counter for the various forked processes snapshotid. it is incremented and set in a persistently shared record
+*/
static ucontext_t savedSnapshotContext;
static ucontext_t savedUserSnapshotContext;
static snapshot_id snapshotid = 0;
}
/** The initSnapShotRecord method initialized the snapshotting data
- * structures for the mprotect based snapshot.
+ * structures for the mprotect based snapshot.
*/
static void initSnapShotRecord(unsigned int numbackingpages, unsigned int numsnapshots, unsigned int nummemoryregions) {
snapshotrecord=( struct SnapShot * )MYMALLOC(sizeof(struct SnapShot));
sTheRecord->mStackSize = STACK_SIZE_DEFAULT;
sTheRecord->mIDToRollback = -1;
sTheRecord->currSnapShotID = 0;
- sTheRecord->mbFinalize = false;
#endif
}
createSharedLibrary();
//step 2 setup the stack context.
-
- int alreadySwapped = 0;
- getcontext( &savedSnapshotContext );
- if( !alreadySwapped ){
- alreadySwapped = 1;
- ucontext_t currentContext, swappedContext, newContext;
- getcontext( &newContext );
- newContext.uc_stack.ss_sp = sTheRecord->mStackBase;
- newContext.uc_stack.ss_size = STACK_SIZE_DEFAULT;
- newContext.uc_link = ¤tContext;
- makecontext( &newContext, entryPoint, 0 );
- swapcontext( &swappedContext, &newContext );
- }
-
- //add the code to take a snapshot here...
- //to return to user process, do a second swapcontext...
+ ucontext_t newContext;
+ getcontext( &newContext );
+ newContext.uc_stack.ss_sp = sTheRecord->mStackBase;
+ newContext.uc_stack.ss_size = STACK_SIZE_DEFAULT;
+ makecontext( &newContext, entryPoint, 0 );
+ /* switch to a new entryPoint context, on a new stack */
+ swapcontext(&savedSnapshotContext, &newContext);
+
+ /* switch back here when takesnapshot is called */
pid_t forkedID = 0;
snapshotid = sTheRecord->currSnapShotID;
bool swapContext = false;
- while( !sTheRecord->mbFinalize ){
+ while( true ){
sTheRecord->currSnapShotID=snapshotid+1;
forkedID = fork();
if( 0 == forkedID ){
ucontext_t currentContext;
-#if 0
- int dbg = 0;
- while( !dbg );
-#endif
+ /*If the rollback bool is set{ see below }, switch to the context we need to return to during a rollback*/
if( swapContext )
swapcontext( ¤tContext, &( sTheRecord->mContextToRollback ) );
else{
+ /*Child process which is forked as a result of takesnapshot call should switch back to the takesnapshot context*/
swapcontext( ¤tContext, &savedUserSnapshotContext );
}
} else {
if( sTheRecord->mIDToRollback != snapshotid )
exit(EXIT_SUCCESS);
else{
+ /*This bool indicates that the current process's snapshotid is same as the id to which the rollback needs to occur*/
swapContext = true;
}
}
#endif
}
-/** The addMemoryRegionToSnapShot function assumes that addr is page aligned.
+/** The addMemoryRegionToSnapShot function assumes that addr is page aligned.
*/
void addMemoryRegionToSnapShot( void * addr, unsigned int numPages) {
#if USE_MPROTECT_SNAPSHOT
sTheRecord->mIDToRollback = theID;
int sTemp = 0;
getcontext( &sTheRecord->mContextToRollback );
+ /*
+ * This is used to quit the process on rollback, so that
+ * the process which needs to rollback can quit allowing the process whose snapshotid matches the rollbackid to switch to
+ * this context and continue....
+ */
if( !sTemp ){
sTemp = 1;
SSDEBUG("Invoked rollback");
exit(EXIT_SUCCESS);
}
+ /*
+ * This fix obviates the need for a finalize call. hence less dependences for model-checker....
+ *
+ */
+ sTheRecord->mIDToRollback = -1;
#endif
}
-/** The finalize method shuts down the snapshotting system. */
-//Subramanian -- remove this function from the external interface and
-//have us call it internally
-void finalize(){
-#if !USE_MPROTECT_SNAPSHOT
- sTheRecord->mbFinalize = true;
-#endif
-}