diff --git a/kernel/console.c b/kernel/console.c
index 23a2d35..b8fa1de 100644
--- a/kernel/console.c
+++ b/kernel/console.c
@@ -89,7 +89,7 @@ consoleread(int user_dst, uint64 dst, int n)
     // wait until interrupt handler has put some
     // input into cons.buffer.
     while(cons.r == cons.w){
-      if(myproc()->killed){
+      if(__sync_add_and_fetch(&(myproc()->killed), 0)){
         release(&cons.lock);
         return -1;
       }
diff --git a/kernel/pipe.c b/kernel/pipe.c
index b6eefb9..e438d7e 100644
--- a/kernel/pipe.c
+++ b/kernel/pipe.c
@@ -81,7 +81,7 @@ pipewrite(struct pipe *pi, uint64 addr, int n)
 
   acquire(&pi->lock);
   while(i < n){
-    if(pi->readopen == 0 || pr->killed){
+    if(pi->readopen == 0 || __sync_add_and_fetch(&pr->killed,0)){
       release(&pi->lock);
       return -1;
     }
@@ -111,7 +111,7 @@ piperead(struct pipe *pi, uint64 addr, int n)
 
   acquire(&pi->lock);
   while(pi->nread == pi->nwrite && pi->writeopen){  //DOC: pipe-empty
-    if(pr->killed){
+    if(__sync_add_and_fetch(&pr->killed,0)){
       release(&pi->lock);
       return -1;
     }
diff --git a/kernel/proc.c b/kernel/proc.c
index 2d0ffa1..221f0f8 100644
--- a/kernel/proc.c
+++ b/kernel/proc.c
@@ -422,7 +422,7 @@ wait(uint64 addr)
     }
 
     // No point waiting if we don't have any children.
-    if(!havekids || p->killed){
+    if(!havekids || __sync_add_and_fetch(&p->killed, 0)){
       release(&wait_lock);
       return -1;
     }
@@ -588,7 +588,7 @@ kill(int pid)
   for(p = proc; p < &proc[NPROC]; p++){
     acquire(&p->lock);
     if(p->pid == pid){
-      p->killed = 1;
+      __sync_bool_compare_and_swap(&p->killed, 0, 1);
       if(p->state == SLEEPING){
         // Wake process from sleep().
         p->state = RUNNABLE;
diff --git a/kernel/sysproc.c b/kernel/sysproc.c
index e8bcda9..61b715b 100644
--- a/kernel/sysproc.c
+++ b/kernel/sysproc.c
@@ -63,7 +63,7 @@ sys_sleep(void)
   acquire(&tickslock);
   ticks0 = ticks;
   while(ticks - ticks0 < n){
-    if(myproc()->killed){
+    if(__sync_add_and_fetch(&(myproc()->killed), 0)){
       release(&tickslock);
       return -1;
     }
diff --git a/kernel/trap.c b/kernel/trap.c
index 75fb3ec..b879f01 100644
--- a/kernel/trap.c
+++ b/kernel/trap.c
@@ -53,7 +53,7 @@ usertrap(void)
   if(r_scause() == 8){
     // system call
 
-    if(p->killed)
+    if(__sync_add_and_fetch(&p->killed, 0))
       exit(-1);
 
     // sepc points to the ecall instruction,
@@ -70,10 +70,10 @@ usertrap(void)
   } else {
     printf("usertrap(): unexpected scause %p pid=%d\n", r_scause(), p->pid);
     printf("            sepc=%p stval=%p\n", r_sepc(), r_stval());
-    p->killed = 1;
+    __sync_bool_compare_and_swap(&p->killed, 0, 1);
   }
 
-  if(p->killed)
+  if(__sync_add_and_fetch(&p->killed, 0))
     exit(-1);
 
   // give up the CPU if this is a timer interrupt.