bio.c

// Buffer cache.
//
// The buffer cache is a linked list of buf structures holding
// cached copies of disk block contents.  Caching disk blocks
// in memory reduces the number of disk reads and also provides
// a synchronization point for disk blocks used by multiple processes.
//
// Interface:
// * To get a buffer for a particular disk block, call bread.
// * After changing buffer data, call bwrite to write it to disk.
// * When done with the buffer, call brelse.
// * 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 three state flags internally:
// * B_BUSY: the block has been returned from bread
//     and has not been passed back to brelse.
// * 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 <sys/types.h>
#include <xv6/param.h>
#include "defs.h"
#include "spinlock.h"
#include "buf.h"

struct {
  struct spinlock lock;
  struct buf buf[NBUF];

  // Linked list of all buffers, through prev/next.
  // head.next is most recently used.
  struct buf head;
} bcache;

void
binit(void)
{
  struct buf *b;

  initlock(&bcache.lock, "bcache");

  // Create linked list of buffers
  bcache.head.prev = &bcache.head;
  bcache.head.next = &bcache.head;
  for(b = bcache.buf; b < bcache.buf+NBUF; b++){
    b->next = bcache.head.next;
    b->prev = &bcache.head;
    b->dev = -1;
    bcache.head.next->prev = b;
    bcache.head.next = b;
  }
}

// Look through buffer cache for sector on device dev.
// If not found, allocate a buffer.
// In either case, return B_BUSY buffer.
static struct buf*
bget(uint dev, uint sector)
{
  struct buf *b;

  acquire(&bcache.lock);

 loop:
  // Is the sector already cached?
  for(b = bcache.head.next; b != &bcache.head; b = b->next){
    if(b->dev == dev && b->sector == sector){
      if(!(b->flags & B_BUSY)){
        b->flags |= B_BUSY;
        release(&bcache.lock);
        return b;
      }
      sleep(b, &bcache.lock);
      goto loop;
    }
  }

  // Not cached; recycle some non-busy and clean buffer.
  // "clean" because B_DIRTY and !B_BUSY means log.c
  // hasn't yet committed the changes to the buffer.
  for(b = bcache.head.prev; b != &bcache.head; b = b->prev){
    if((b->flags & B_BUSY) == 0 && (b->flags & B_DIRTY) == 0){
      b->dev = dev;
      b->sector = sector;
      b->flags = B_BUSY;
      release(&bcache.lock);
      return b;
    }
  }
  panic("bget: no buffers");
}

// Return a B_BUSY buf with the contents of the indicated disk sector.
struct buf*
bread(uint dev, uint sector)
{
  struct buf *b;

  b = bget(dev, sector);
  if(!(b->flags & B_VALID))
    iderw(b);
  return b;
}

// Write b's contents to disk.  Must be B_BUSY.
void
bwrite(struct buf *b)
{
  if((b->flags & B_BUSY) == 0)
    panic("bwrite");
  b->flags |= B_DIRTY;
  iderw(b);
}

// Release a B_BUSY buffer.
// Move to the head of the MRU list.
void
brelse(struct buf *b)
{
  if((b->flags & B_BUSY) == 0)
    panic("brelse");

  acquire(&bcache.lock);

  b->next->prev = b->prev;
  b->prev->next = b->next;
  b->next = bcache.head.next;
  b->prev = &bcache.head;
  bcache.head.next->prev = b;
  bcache.head.next = b;

  b->flags &= ~B_BUSY;
  wakeup(b);

  release(&bcache.lock);
}