diff --git a/Makefile b/Makefile
index 6ba1d6d..b50dca0 100644
--- a/Makefile
+++ b/Makefile
@@ -61,7 +61,7 @@ endif
 
 # riscv64-unknown-elf- or riscv64-linux-gnu-
 # perhaps in /opt/riscv/bin
-#TOOLPREFIX = 
+TOOLPREFIX = /opt/riscv/bin/riscv64-unknown-elf-
 
 # Try to infer the correct TOOLPREFIX if not set
 ifndef TOOLPREFIX
diff --git a/fs.img.bk b/fs.img.bk
new file mode 100644
index 0000000..2bd36ab
Binary files /dev/null and b/fs.img.bk differ
diff --git a/kernel/kalloc.c b/kernel/kalloc.c
index 0699e7e..7a09849 100644
--- a/kernel/kalloc.c
+++ b/kernel/kalloc.c
@@ -21,12 +21,13 @@ struct run {
 struct {
   struct spinlock lock;
   struct run *freelist;
-} kmem;
+} kmem[NCPU];
 
-void
+void 
 kinit()
 {
-  initlock(&kmem.lock, "kmem");
+  for(int i=0;i<NCPU;++i)
+    initlock(&kmem[i].lock, "kmem");
   freerange(end, (void*)PHYSTOP);
 }
 
@@ -45,7 +46,9 @@ freerange(void *pa_start, void *pa_end)
 // initializing the allocator; see kinit above.)
 void
 kfree(void *pa)
-{
+{ 
+  push_off();
+  const int cpu=cpuid();
   struct run *r;
 
   if(((uint64)pa % PGSIZE) != 0 || (char*)pa < end || (uint64)pa >= PHYSTOP)
@@ -55,11 +58,11 @@ kfree(void *pa)
   memset(pa, 1, PGSIZE);
 
   r = (struct run*)pa;
-
-  acquire(&kmem.lock);
-  r->next = kmem.freelist;
-  kmem.freelist = r;
-  release(&kmem.lock);
+  acquire(&kmem[cpu].lock);
+  r->next = kmem[cpu].freelist;
+  kmem[cpu].freelist = r;
+  release(&kmem[cpu].lock);
+  pop_off();
 }
 
 // Allocate one 4096-byte page of physical memory.
@@ -68,15 +71,33 @@ kfree(void *pa)
 void *
 kalloc(void)
 {
+  push_off();
   struct run *r;
-
-  acquire(&kmem.lock);
-  r = kmem.freelist;
+  const int cpu=cpuid();
+  acquire(&kmem[cpu].lock);
+  r = kmem[cpu].freelist;
   if(r)
-    kmem.freelist = r->next;
-  release(&kmem.lock);
-
+    kmem[cpu].freelist = r->next;
+  while(!r)
+    for(int i=0;i<NCPU;++i){
+      if(i==cpu)
+        continue;
+      if(kmem[i].lock.locked)
+        continue;
+      acquire(&kmem[i].lock);
+      r=kmem[i].freelist;
+      if(r){
+        kmem[i].freelist = r->next;
+        goto fine;
+      }
+      release(&kmem[i].lock);
+      continue;
+      fine:
+      break;
+    }
+  release(&kmem[cpu].lock);
   if(r)
     memset((char*)r, 5, PGSIZE); // fill with junk
+  pop_off();
   return (void*)r;
 }