Adding relevant comments for fork based implementation.

diff --git a/snapshot.cc b/snapshot.cc
index 5fb1a85ebd4c4e633e1da3812b97776a893dccd8..1e7d80b4b5d836d1c4a1cdb653824a626afb82c8 100644 (file)
--- 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;
@@ -185,8 +196,7 @@ void initSnapShotLibrary(unsigned int numbackingpages,
        /* switch to a new entryPoint context, on a new stack */
        swapcontext(&savedSnapshotContext, &newContext);
 
-       //add the code to take a snapshot here...
-       //to return to user process, do a second swapcontext...
+       /* switch back here when takesnapshot is called */
        pid_t forkedID = 0;
        snapshotid = sTheRecord->currSnapShotID;
        bool swapContext = false;
@@ -195,13 +205,11 @@ void initSnapShotLibrary(unsigned int numbackingpages,
                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 {
@@ -218,6 +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;
                        }
                }
@@ -299,11 +308,20 @@ 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
 }