X-Git-Url: http://plrg.eecs.uci.edu/git/?p=c11tester.git;a=blobdiff_plain;f=snapshot.cc;h=1e7d80b4b5d836d1c4a1cdb653824a626afb82c8;hp=9a69a4b310141cd8fafd8fde5a2b45691b9c9b02;hb=c9719c5b5bebeed83f9864825fa93ff6bae2b575;hpb=84b27b559dac866588ee23920b930d5c58684815

diff --git a/snapshot.cc b/snapshot.cc
index 9a69a4b3..1e7d80b4 100644
--- a/snapshot.cc
+++ b/snapshot.cc
@@ -33,6 +33,17 @@ struct Snapshot * sTheRecord = NULL;
 #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;
@@ -120,7 +131,6 @@ void createSharedLibrary(){
 	sTheRecord->mStackSize = STACK_SIZE_DEFAULT;
 	sTheRecord->mIDToRollback = -1;
 	sTheRecord->currSnapShotID = 0;
-	sTheRecord->mbFinalize = false;
 #endif
 }
 
@@ -178,37 +188,28 @@ void initSnapShotLibrary(unsigned int numbackingpages,
 	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 = &currentContext;
-		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( &currentContext, &( sTheRecord->mContextToRollback ) );
 			else{
+				/*Child process which is forked as a result of takesnapshot call should switch back to the takesnapshot context*/
 				swapcontext( &currentContext, &savedUserSnapshotContext );
 			}
 		} else {
@@ -225,6 +226,7 @@ void initSnapShotLibrary(unsigned int numbackingpages,
 			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;
 			}
 		}
@@ -306,19 +308,21 @@ void rollBack( snapshot_id theID ){
 	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
-}