diff --git a/kernel/bio.c b/kernel/bio.c
index a08884b..a1074f2 100644
--- a/kernel/bio.c
+++ b/kernel/bio.c
@@ -12,11 +12,7 @@
 // * Do not use the buffer after calling brelse.
 // * Only one process at a time can use a buffer,
 //     so do not keep them longer than necessary.
-//
-// The implementation uses two state flags internally:
-// * B_VALID: the buffer data has been read from the disk.
-// * B_DIRTY: the buffer data has been modified
-//     and needs to be written to disk.
+
 
 #include "types.h"
 #include "param.h"
@@ -76,13 +72,11 @@ bget(uint dev, uint blockno)
   }
 
   // Not cached; recycle an unused buffer.
-  // Even if refcnt==0, B_DIRTY indicates a buffer is in use
-  // because log.c has modified it but not yet committed it.
   for(b = bcache.head.prev; b != &bcache.head; b = b->prev){
-    if(b->refcnt == 0 && (b->flags & B_DIRTY) == 0) {
+    if(b->refcnt == 0) {
       b->dev = dev;
       b->blockno = blockno;
-      b->flags = 0;
+      b->valid = 0;
       b->refcnt = 1;
       release(&bcache.lock);
       acquiresleep(&b->lock);
@@ -99,8 +93,9 @@ bread(uint dev, uint blockno)
   struct buf *b;
 
   b = bget(dev, blockno);
-  if((b->flags & B_VALID) == 0) {
-    virtio_disk_rw(b);
+  if(!b->valid) {
+    virtio_disk_rw(b, 0);
+    b->valid = 1;
   }
   return b;
 }
@@ -111,8 +106,7 @@ bwrite(struct buf *b)
 {
   if(!holdingsleep(&b->lock))
     panic("bwrite");
-  b->flags |= B_DIRTY;
-  virtio_disk_rw(b);
+  virtio_disk_rw(b, 1);
 }
 
 // Release a locked buffer.
@@ -139,3 +133,19 @@ brelse(struct buf *b)
   
   release(&bcache.lock);
 }
+
+void
+bpin(struct buf *b) {
+  acquire(&bcache.lock);
+  b->refcnt++;
+  release(&bcache.lock);
+}
+
+void
+bunpin(struct buf *b) {
+  acquire(&bcache.lock);
+  b->refcnt--;
+  release(&bcache.lock);
+}
+
+
diff --git a/kernel/buf.h b/kernel/buf.h
index 3266495..4a3a39d 100644
--- a/kernel/buf.h
+++ b/kernel/buf.h
@@ -1,5 +1,6 @@
 struct buf {
-  int flags;
+  int valid;   // has data been read from disk?
+  int disk;    // does disk "own" buf?
   uint dev;
   uint blockno;
   struct sleeplock lock;
@@ -9,6 +10,4 @@ struct buf {
   struct buf *qnext; // disk queue
   uchar data[BSIZE];
 };
-#define B_VALID 0x2  // buffer has been read from disk
-#define B_DIRTY 0x4  // buffer needs to be written to disk
 
diff --git a/kernel/defs.h b/kernel/defs.h
index de4acfd..2689bed 100644
--- a/kernel/defs.h
+++ b/kernel/defs.h
@@ -14,6 +14,8 @@ void            binit(void);
 struct buf*     bread(uint, uint);
 void            brelse(struct buf*);
 void            bwrite(struct buf*);
+void            bpin(struct buf*);
+void            bunpin(struct buf*);
 
 // console.c
 void            consoleinit(void);
@@ -177,7 +179,7 @@ void            plic_complete(int);
 
 // virtio_disk.c
 void            virtio_disk_init(void);
-void            virtio_disk_rw(struct buf *);
+void            virtio_disk_rw(struct buf *, int);
 void            virtio_disk_intr();
 
 // number of elements in fixed-size array
diff --git a/kernel/log.c b/kernel/log.c
index c8f7e62..59984db 100644
--- a/kernel/log.c
+++ b/kernel/log.c
@@ -77,6 +77,7 @@ install_trans(void)
     struct buf *dbuf = bread(log.dev, log.lh.block[tail]); // read dst
     memmove(dbuf->data, lbuf->data, BSIZE);  // copy block to dst
     bwrite(dbuf);  // write dst to disk
+    bunpin(dbuf);
     brelse(lbuf);
     brelse(dbuf);
   }
@@ -203,7 +204,7 @@ commit()
 }
 
 // Caller has modified b->data and is done with the buffer.
-// Record the block number and pin in the cache with B_DIRTY.
+// Record the block number and pin in the cache by increasing refcnt.
 // commit()/write_log() will do the disk write.
 //
 // log_write() replaces bwrite(); a typical use is:
@@ -227,9 +228,10 @@ log_write(struct buf *b)
       break;
   }
   log.lh.block[i] = b->blockno;
-  if (i == log.lh.n)
+  if (i == log.lh.n) {  // Add new block to log?
+    bpin(b);
     log.lh.n++;
-  b->flags |= B_DIRTY; // prevent eviction
+  }
   release(&log.lock);
 }
 
diff --git a/kernel/virtio_disk.c b/kernel/virtio_disk.c
index 3ace5ef..14c718d 100644
--- a/kernel/virtio_disk.c
+++ b/kernel/virtio_disk.c
@@ -164,7 +164,7 @@ alloc3_desc(int *idx)
 }
 
 void
-virtio_disk_rw(struct buf *b)
+virtio_disk_rw(struct buf *b, int write)
 {
   uint64 sector = b->blockno * (BSIZE / 512);
 
@@ -192,7 +192,7 @@ virtio_disk_rw(struct buf *b)
     uint64 sector;
   } buf0;
 
-  if(b->flags & B_DIRTY)
+  if(write)
     buf0.type = VIRTIO_BLK_T_OUT; // write the disk
   else
     buf0.type = VIRTIO_BLK_T_IN; // read the disk
@@ -208,7 +208,7 @@ virtio_disk_rw(struct buf *b)
 
   desc[idx[1]].addr = (uint64) b->data;
   desc[idx[1]].len = BSIZE;
-  if(b->flags & B_DIRTY)
+  if(write)
     desc[idx[1]].flags = 0; // device reads b->data
   else
     desc[idx[1]].flags = VRING_DESC_F_WRITE; // device writes b->data
@@ -222,6 +222,7 @@ virtio_disk_rw(struct buf *b)
   desc[idx[2]].next = 0;
 
   // record struct buf for virtio_disk_intr().
+  b->disk = 1;
   info[idx[0]].b = b;
 
   // avail[0] is flags
@@ -235,10 +236,13 @@ virtio_disk_rw(struct buf *b)
   *R(VIRTIO_MMIO_QUEUE_NOTIFY) = 0; // value is queue number
 
   // Wait for virtio_disk_intr() to say request has finished.
-  while((b->flags & (B_VALID|B_DIRTY)) != B_VALID){
+  while(b->disk == 1) {
     sleep(b, &vdisk_lock);
   }
 
+  info[idx[0]].b = 0;
+  free_chain(idx[0]);
+
   release(&vdisk_lock);
 }
 
@@ -252,15 +256,10 @@ virtio_disk_intr()
 
     if(info[id].status != 0)
       panic("virtio_disk_intr status");
-
-    info[id].b->flags |= B_VALID;
-    info[id].b->flags &= ~B_DIRTY;
-
+    
+    info[id].b->disk = 0;   // disk is done with buf
     wakeup(info[id].b);
 
-    info[id].b = 0;
-    free_chain(id);
-
     used_idx = (used_idx + 1) % NUM;
   }
 
diff --git a/labs/lock.html b/labs/lock.html
index b43a51b..707d6c4 100644
--- a/labs/lock.html
+++ b/labs/lock.html
@@ -82,7 +82,7 @@ workloads.
 
 <p>Run usertests to see if you don't break anything.
 
-<h2>Lock-free bcache lookup</h2>
+<h2>More scalabale bcache lookup</h2>
 
 
 <p>Several processes reading different files repeatedly will
@@ -98,7 +98,7 @@ workloads.
   and run test0 from bcachetest and you will see "!"s.
 
 <p>Modify <tt>bget</tt> so that a lookup for a buffer that is in the
-  bcache doesn't need to acquire <tt>bcache.lock</tt>.  This more
+  bcache doesn't need to acquire <tt>bcache.lock</tt>.  This is more
   tricky than the kalloc assignment, because bcache buffers are truly
   shared among processes. You must maintain the invariant that a
   buffer is only once in memory.
@@ -116,15 +116,33 @@ against, including:
 
 <p>A challenge is testing whether you code is still correct.  One way
   to do is to artificially delay certain operations
-  using <tt>sleepticks</tt>.
+  using <tt>sleepticks</tt>.  <tt>test1</tt> trashes the buffer cache
+  and exercises more code paths.
 
 <p>Here are some hints:
   <ul>
-    <li> Use an atomic increment instruction for incrementing and
-      decrementing <tt>b->ref</tt> (see <tt>__sync_fetch_and_add() and
-	related primitives</tt>)
-    <li>Don't walk the <tt>bcache.head</tt> list to find a buffer
+    <li>Read the description of buffer cache in the xv6 book (Section 7.2).
+    <li>Use a simple design: i.e., don't design a lock-free implementation.
+    <li>Use a simple hash table with locks per bucket.
+    <li>Searching in hash table for a buffer and allocating an entry
+      for that buffer when the buffer is not found must be atomic.
+    <li>It is fine to acquire <tt>bcache.lock</tt> in <tt>brelse</tt>
+      to update the LRU/MRU list.
   </ul>
+
+<p>Check that your implementation has less contention
+  on <tt>test0</tt>
+
+<p>Make sure your implementation passes bcachetest and usertests.
+
+<p>Optional:
+  <ul>
+  <li>make the buffer cache more scalable (e.g., avoid taking
+  out <tt>bcache.lock</tt> on <tt>brelse</tt>).
+  <li>make lookup lock-free (Hint: use gcc's <tt>__sync_*</tt>
+    functions.) How do you convince yourself that your implementation is correct?
+  </ul>
+  
   
 </body>
 </html>