xv6/bio.c
Frans Kaashoek af03ab142d a few nits
passes all tests on two-processor smp
2010-07-23 12:52:50 -04:00

139 lines
3.1 KiB
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 {
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");
//PAGEBREAK!
// 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 fresh block.
// In either case, return locked buffer.
static struct buf*
bget(uint dev, uint sector)
{
struct buf *b;
acquire(&bcache.lock);
loop:
// Try for cached block.
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;
}
}
// Allocate fresh block.
for(b = bcache.head.prev; b != &bcache.head; b = b->prev){
if((b->flags & B_BUSY) == 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 locked.
void
bwrite(struct buf *b)
{
if((b->flags & B_BUSY) == 0)
panic("bwrite");
b->flags |= B_DIRTY;
iderw(b);
}
// Release the buffer b.
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);
}