diff --git a/Makefile b/Makefile
index ba7a6fb..7229d5e 100644
--- a/Makefile
+++ b/Makefile
@@ -104,7 +104,7 @@ $U/_forktest: $U/forktest.o $(ULIB)
 	$(LD) $(LDFLAGS) -N -e main -Ttext 0 -o $U/_forktest $U/forktest.o $U/ulib.o $U/usys.o
 	$(OBJDUMP) -S $U/_forktest > $U/forktest.asm
 
-mkfs/mkfs: mkfs/mkfs.c $K/fs.h
+mkfs/mkfs: mkfs/mkfs.c $K/fs.h $K/param.h
 	gcc -Werror -Wall -I. -o mkfs/mkfs mkfs/mkfs.c
 
 # Prevent deletion of intermediate files, e.g. cat.o, after first build, so
diff --git a/kernel/riscv.h b/kernel/riscv.h
index f46ba59..0aec003 100644
--- a/kernel/riscv.h
+++ b/kernel/riscv.h
@@ -261,7 +261,6 @@ r_time()
 static inline void
 intr_on()
 {
-  w_sie(r_sie() | SIE_SEIE | SIE_STIE | SIE_SSIE);
   w_sstatus(r_sstatus() | SSTATUS_SIE);
 }
 
diff --git a/kernel/spinlock.c b/kernel/spinlock.c
index f192832..9840302 100644
--- a/kernel/spinlock.c
+++ b/kernel/spinlock.c
@@ -72,13 +72,12 @@ release(struct spinlock *lk)
 }
 
 // Check whether this cpu is holding the lock.
+// Interrupts must be off.
 int
 holding(struct spinlock *lk)
 {
   int r;
-  push_off();
   r = (lk->locked && lk->cpu == mycpu());
-  pop_off();
   return r;
 }
 
@@ -103,9 +102,9 @@ pop_off(void)
   struct cpu *c = mycpu();
   if(intr_get())
     panic("pop_off - interruptible");
-  c->noff -= 1;
-  if(c->noff < 0)
+  if(c->noff < 1)
     panic("pop_off");
+  c->noff -= 1;
   if(c->noff == 0 && c->intena)
     intr_on();
 }
diff --git a/kernel/start.c b/kernel/start.c
index 203c5e6..4eb6c2d 100644
--- a/kernel/start.c
+++ b/kernel/start.c
@@ -36,6 +36,7 @@ start()
   // delegate all interrupts and exceptions to supervisor mode.
   w_medeleg(0xffff);
   w_mideleg(0xffff);
+  w_sie(r_sie() | SIE_SEIE | SIE_STIE | SIE_SSIE);
 
   // ask for clock interrupts.
   timerinit();
diff --git a/kernel/trap.c b/kernel/trap.c
index ca732f2..5e11e4b 100644
--- a/kernel/trap.c
+++ b/kernel/trap.c
@@ -129,7 +129,6 @@ usertrapret(void)
 
 // interrupts and exceptions from kernel code go here via kernelvec,
 // on whatever the current kernel stack is.
-// must be 4-byte aligned to fit in stvec.
 void 
 kerneltrap()
 {
@@ -189,9 +188,13 @@ devintr()
       uartintr();
     } else if(irq == VIRTIO0_IRQ){
       virtio_disk_intr();
+    } else if(irq){
+      printf("unexpected interrupt irq=%d\n", irq);
     }
 
-    plic_complete(irq);
+    if(irq)
+      plic_complete(irq);
+
     return 1;
   } else if(scause == 0x8000000000000001L){
     // software interrupt from a machine-mode timer interrupt,
diff --git a/user/usertests.c b/user/usertests.c
index db9f680..eb10ee2 100644
--- a/user/usertests.c
+++ b/user/usertests.c
@@ -2105,9 +2105,9 @@ run(void f(char *), char *s) {
   } else {
     wait(&xstatus);
     if(xstatus != 0) 
-      printf("FAILED\n", s);
+      printf("FAILED\n");
     else
-      printf("OK\n", s);
+      printf("OK\n");
     return xstatus == 0;
   }
 }