Find file
Fetching contributors…
Cannot retrieve contributors at this time
138 lines (118 sloc) 3.11 KB
// 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");
// 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))
ide_rw(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;
ide_rw(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);
}