diff --git a/arch/x86/kernel/gencore.c b/arch/x86/kernel/gencore.c
index 9bde3b62..5cb4baa6 100644
--- a/arch/x86/kernel/gencore.c
+++ b/arch/x86/kernel/gencore.c
@@ -9,7 +9,7 @@
 #define	align32(x) ((((x) + 3) / 4) * 4)
 #define	alignpage(x) ((((x) + (PAGE_SIZE) - 1) / (PAGE_SIZE)) * (PAGE_SIZE))
 
-#define DEBUG_PRINT_GENCORE
+//#define DEBUG_PRINT_GENCORE
 
 #ifdef DEBUG_PRINT_GENCORE
 #define	dkprintf(...)	kprintf(__VA_ARGS__)
diff --git a/arch/x86/kernel/syscall.c b/arch/x86/kernel/syscall.c
index 92b8be1e..b9059f9f 100644
--- a/arch/x86/kernel/syscall.c
+++ b/arch/x86/kernel/syscall.c
@@ -207,6 +207,7 @@ do_signal(unsigned long rc, void *regs0, struct process *proc, struct sig_pendin
 	int	irqstate;
 
 	for(w = pending->sigmask.__val[0], sig = 0; w; sig++, w >>= 1);
+	dkprintf("do_signal,pid=%d,sig=%d\n", proc->pid, sig);
 
 	if(regs == NULL){ /* call from syscall */
 		asm("movq %%gs:132, %0" : "=r" (regs));
@@ -313,13 +314,10 @@ do_signal(unsigned long rc, void *regs0, struct process *proc, struct sig_pendin
 			/* Wake up the parent who tried wait4 and sleeping */
 			waitq_wakeup(&proc->ftn->parent->waitpid_q);
 
-			dkprintf("do_signal,SIGTRAP,sleeping\n");
 			/* Sleep */
+			dkprintf("do_signal,SIGTRAP,sleeping\n");
 			proc->status = PS_TRACED;
 
-			//struct cpu_local_var *v = get_this_cpu_local_var();
-			//v->flags |= CPU_FLAG_NEED_RESCHED;
-
 			schedule();
 			dkprintf("SIGTRAP(): woken up\n");
 			break; }
@@ -357,6 +355,8 @@ static int ptrace_report_signal(struct process *proc, struct sig_pending *pendin
 	__sigset_t w;
 	long rc;
 
+	dkprintf("ptrace_report_signal,pid=%d\n", proc->pid);
+
 	/* Save reason why stopped and process state for wait to reap */
 	for (w = pending->sigmask.__val[0], sig = 0; w; sig++, w >>= 1);
 	ihk_mc_spinlock_lock_noirq(&proc->ftn->lock);	
@@ -673,10 +673,10 @@ do_kill(int pid, int tid, int sig, siginfo_t *info)
 
 	if(doint && !(mask & tproc->sigmask.__val[0])){
 		switch(sig) {
+		case SIGKILL:
 		case SIGCONT:
 			break;
 		case SIGSTOP:
-		case SIGKILL:
 		default:
 			if(proc != tproc){
 				dkprintf("do_kill,ipi,pid=%d,cpu_id=%d\n",
@@ -697,6 +697,13 @@ do_kill(int pid, int tid, int sig, siginfo_t *info)
 					tproc->pid, tproc->cpu_id);
 			interrupt_syscall(tproc->pid, tproc->cpu_id);
 #endif
+			/* Wake up the target only when stopped by ptrace-reporting */
+			sched_wakeup_process(tproc, PS_TRACED);
+			ihk_mc_spinlock_lock_noirq(&tproc->ftn->lock);	
+			if (tproc->ftn->status & PS_TRACED) {
+				xchg4((int *)(&tproc->ftn->status), PS_RUNNING);
+			} 
+			ihk_mc_spinlock_unlock_noirq(&tproc->ftn->lock);
 			break;
 		case SIGCONT:
 			/* Wake up the target only when stopped by SIGSTOP */
diff --git a/kernel/init.c b/kernel/init.c
index 39deea57..66a808e2 100644
--- a/kernel/init.c
+++ b/kernel/init.c
@@ -141,7 +141,7 @@ void pc_init(void)
 			osnum += *p++ - '0';
 		}
 	}
-	kprintf("osnum: %d\n", osnum);
+	dkprintf("osnum: %d\n", osnum);
 
 
 	if (!(p = find_command_line("perfctr"))) {
diff --git a/kernel/process.c b/kernel/process.c
index 4ae6473c..abfaf748 100644
--- a/kernel/process.c
+++ b/kernel/process.c
@@ -48,6 +48,11 @@ static void insert_vm_range_list(struct process_vm *vm,
 static int copy_user_ranges(struct process *proc, struct process *org);
 void settid(struct process *proc, int mode, int newcpuid, int oldcpuid);
 
+int refcount_fork_tree_node(struct fork_tree_node *ftn)
+{
+	return ihk_atomic_read(&ftn->refcount);
+}
+
 void hold_fork_tree_node(struct fork_tree_node *ftn)
 {
 	ihk_atomic_inc(&ftn->refcount);
@@ -318,23 +323,48 @@ err_free_proc:
 }
 
 int ptrace_traceme(void){
+	int error = 0;
 	struct process *proc = cpu_local_var(current);
-	struct fork_tree_node *ftn = proc->ftn, *parent;
+	struct fork_tree_node *child, *next;
+	dkprintf("ptrace_traceme,pid=%d,proc->ftn->parent=%p\n", proc->pid, proc->ftn->parent);
 
-	ftn->ptrace = PT_TRACED;
-
-	parent = ftn->parent;
-	if (parent != NULL) {
-		ftn->ppid_parent = parent;
-
-		ihk_mc_spinlock_lock_noirq(&parent->lock);
-		list_add_tail(&ftn->ptrace_siblings_list, &parent->ptrace_children);
-		ihk_mc_spinlock_unlock_noirq(&parent->lock);
+	if (proc->ftn->parent == NULL) {
+		error = -EPERM;
+		goto out;
 	}
 
-	return 0;
-}
+	dkprintf("ptrace_traceme,parent->pid=%d\n", proc->ftn->parent->pid);
+	
+	ihk_mc_spinlock_lock_noirq(&proc->ftn->lock);
+	
+	proc->ftn->ptrace = PT_TRACED;
+	proc->ftn->ppid_parent = proc->ftn->parent;
+	
+	ihk_mc_spinlock_lock_noirq(&proc->ftn->parent->lock);
+	list_for_each_entry_safe(child, next, &proc->ftn->parent->children, siblings_list) {
+		if(child == proc->ftn) {
+			list_del(&child->siblings_list);
+			goto found;
+		}
+	}
+	kprintf("ptrace_traceme,not found\n");
+	error = -EPERM;
+	goto out_notfound;
+ found:
+	list_add_tail(&proc->ftn->ptrace_siblings_list, &proc->ftn->parent->ptrace_children);
+	
+	ihk_mc_spinlock_unlock_noirq(&proc->ftn->parent->lock);
+	
+	ihk_mc_spinlock_unlock_noirq(&proc->ftn->lock);
 
+ out:
+	dkprintf("ptrace_traceme,returning,error=%d\n", error);
+	return error;
+ out_notfound:
+	ihk_mc_spinlock_unlock_noirq(&proc->ftn->parent->lock);
+	ihk_mc_spinlock_unlock_noirq(&proc->ftn->lock);
+	goto out;
+}
 
 static int copy_user_ranges(struct process *proc, struct process *org) 
 {
diff --git a/kernel/syscall.c b/kernel/syscall.c
index 7575111a..c0ae4880 100644
--- a/kernel/syscall.c
+++ b/kernel/syscall.c
@@ -301,16 +301,12 @@ static int wait_zombie(struct process *proc, struct fork_tree_node *child, int *
     int ret;
     struct syscall_request request IHK_DMA_ALIGN;
     
-    dkprintf("wait: found PS_ZOMBIE process: %d\n", child->pid);
-    
-    list_del(&child->siblings_list);	
+    dkprintf("wait_zombie,found PS_ZOMBIE process: %d\n", child->pid);
     
     if (status) {
         *status = child->exit_status;
     }
     
-    release_fork_tree_node(child);
-    
     /* Ask host to clean up exited child */
     request.number = __NR_wait4;
     request.args[0] = child->pid;
@@ -401,7 +397,6 @@ SYSCALL_DECLARE(wait4)
 	pid = (int)ihk_mc_syscall_arg0(ctx);	
 
 	ihk_mc_spinlock_lock_noirq(&proc->ftn->lock);
-
 	list_for_each_entry(child_iter, &proc->ftn->children, siblings_list) {	
 
 		ihk_mc_spinlock_lock_noirq(&child_iter->lock);
@@ -416,6 +411,8 @@ SYSCALL_DECLARE(wait4)
 			if(child_iter->status == PS_ZOMBIE) {
 				ret = wait_zombie(proc, child_iter, status, ctx);
 				if(ret) {
+					list_del(&child_iter->siblings_list);
+					release_fork_tree_node(child_iter);
 					goto out_found;
 				}
 			}
@@ -440,7 +437,6 @@ SYSCALL_DECLARE(wait4)
 
 		ihk_mc_spinlock_unlock_noirq(&child_iter->lock);
 	}
-
 	list_for_each_entry(child_iter, &proc->ftn->ptrace_children, ptrace_siblings_list) {	
 
 		ihk_mc_spinlock_lock_noirq(&child_iter->lock);
@@ -455,6 +451,8 @@ SYSCALL_DECLARE(wait4)
 			if(child_iter->status == PS_ZOMBIE) {
 				ret = wait_zombie(proc, child_iter, status, ctx);
 				if(ret) {
+					list_del(&child_iter->ptrace_siblings_list);
+					release_fork_tree_node(child_iter);
 					goto out_found;
 				}
 			}
@@ -510,8 +508,10 @@ SYSCALL_DECLARE(wait4)
  exit:
 	return ret;
  out_found:
+	dkprintf("wait4,out_found\n");
 	ihk_mc_spinlock_unlock_noirq(&child_iter->lock);
  out_notfound:
+	dkprintf("wait4,out_notfound\n");
 	ihk_mc_spinlock_unlock_noirq(&proc->ftn->lock);
 	goto exit;
 }
@@ -526,6 +526,7 @@ terminate(int rc, int sig, ihk_mc_user_context_t *ctx)
 	struct process *parent_owner;
 	int error;
 
+	dkprintf("terminate,pid=%d\n", proc->pid);
 	request.number = __NR_exit_group;
 	request.args[0] = ((rc & 0x00ff) << 8) | (sig & 0xff);
 
@@ -546,10 +547,14 @@ terminate(int rc, int sig, ihk_mc_user_context_t *ctx)
 		list_del(&child->siblings_list);
 		release_fork_tree_node(child);
 	}
+	list_for_each_entry_safe(child, next, &ftn->ptrace_children, ptrace_siblings_list) {
+		list_del(&child->ptrace_siblings_list);
+		release_fork_tree_node(child);
+	}
 	ftn->owner = NULL;
 	ihk_mc_spinlock_unlock_noirq(&ftn->lock);	
 
-	/* Send SIGCHILD to parent */
+	/* Send SIGCHLD to parent */
 	if (ftn->parent) {
 		int parent_owner_pid;
 
@@ -584,19 +589,18 @@ terminate(int rc, int sig, ihk_mc_user_context_t *ctx)
 /*
 			sigchld_parent(ftn->parent->owner, 0);
 */
-			dkprintf("terminate,klll SIGCHILD,error=%d\n",
+			dkprintf("terminate,klll SIGCHLD,error=%d\n",
 					 error);
 		}
-	
+
 		release_fork_tree_node(ftn->parent);
 	} else {
 		ihk_mc_spinlock_lock_noirq(&ftn->lock);
 		ftn->status = PS_EXITED;
 		ihk_mc_spinlock_unlock_noirq(&ftn->lock);	
     }
-
 	release_fork_tree_node(ftn);
-	
+
 	proc->status = PS_EXITED;
 	release_process(proc);
 	
@@ -1565,6 +1569,7 @@ unsigned long do_fork(int clone_flags, unsigned long newsp,
 
 	cpuid = obtain_clone_cpuid();
     if (cpuid == -1) {
+		kprintf("do_fork,core not available\n");
         return -EAGAIN;
     }