Remove B_DIRTY

Use refcnt to pin blocks into the cache
Replace flags/B_VALID with a boolean field valid
Use info[id].status to signal completion of disk interrupt
Pass a read/write flag to virtio_disk_rw

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) {
+  if(!b->valid) {
-    virtio_disk_rw(b);
+    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, 1);
-  virtio_disk_rw(b);
 }
 
 // Release a locked buffer.

kernel/buf.h

@@ -1,5 +1,5 @@
 struct buf {
-  int flags;
+  int valid;   // has data been read from disk?
   uint dev;
   uint blockno;
   struct sleeplock lock;
@@ -9,6 +9,4 @@ 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
 

kernel/defs.h

@@ -177,7 +177,7 @@ 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

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
+    dbuf->refcnt--;  // unpin buffer from cache
     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?
+    b->refcnt++;  // Pin block in cache
     log.lh.n++;
-  b->flags |= B_DIRTY; // prevent eviction
+  }
   release(&log.lock);
 }
 

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
@@ -235,7 +235,7 @@ 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(info[idx[0]].status == 0) {
     sleep(b, &vdisk_lock);
   }
 
@@ -253,9 +253,7 @@ virtio_disk_intr()
     if(info[id].status != 0)
       panic("virtio_disk_intr status");
 
-    info[id].b->flags |= B_VALID;
+    info[id].status = 1;
-    info[id].b->flags &= ~B_DIRTY;
-
     wakeup(info[id].b);
 
     info[id].b = 0;