xv6/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 flush 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 initialized
//     with the associated disk block contents.
// * B_DIRTY: the buffer data has been modified
//     and needs to be written to disk.

#include "types.h"
#include "defs.h"
#include "param.h"
#include "spinlock.h"
#include "buf.h"

struct buf buf[NBUF];
struct spinlock buf_table_lock;

// Linked list of all buffers, through prev/next.
// bufhead->next is most recently used.
// bufhead->tail is least recently used.
struct buf bufhead;

void
binit(void)
{
  struct buf *b;

  initlock(&buf_table_lock, "buf_table");

//PAGEBREAK!
  // Create linked list of buffers
  bufhead.prev = &bufhead;
  bufhead.next = &bufhead;
  for(b = buf; b < buf+NBUF; b++){
    b->next = bufhead.next;
    b->prev = &bufhead;
    bufhead.next->prev = b;
    bufhead.next = b;
  }
}

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

  acquire(&buf_table_lock);

 loop:
  // Try for cached block.
  for(b = bufhead.next; b != &bufhead; b = b->next){
    if((b->flags & (B_BUSY|B_VALID)) &&
       b->dev == dev && b->sector == sector){
      if(b->flags & B_BUSY){
        sleep(buf, &buf_table_lock);
        goto loop;
      }
      b->flags |= B_BUSY;
      release(&buf_table_lock);
      return b;
    }
  }

  // Allocate fresh block.
  for(b = bufhead.prev; b != &bufhead; b = b->prev){
    if((b->flags & B_BUSY) == 0){
      b->flags = B_BUSY;
      b->dev = dev;
      b->sector = sector;
      release(&buf_table_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 buf's contents to disk.  Must be locked.
void
bwrite(struct buf *b)
{
  if((b->flags & B_BUSY) == 0)
    panic("bwrite");
  b->flags |= B_DIRTY;
  iderw(b);
}

// Release the buffer buf.
void
brelse(struct buf *b)
{
  if((b->flags & B_BUSY) == 0)
    panic("brelse");

  acquire(&buf_table_lock);

  b->next->prev = b->prev;
  b->prev->next = b->next;
  b->next = bufhead.next;
  b->prev = &bufhead;
  bufhead.next->prev = b;
  bufhead.next = b;

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

  release(&buf_table_lock);
}