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bio.c
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bio.c
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// 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 "types.h"
#include "defs.h"
#include "param.h"
#include "spinlock.h"
#include "fs.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 block on device dev.
// If not found, allocate a buffer.
// In either case, return B_BUSY buffer.
static struct buf*
bget(uint dev, uint blockno)
{
struct buf *b;
acquire(&bcache.lock);
loop:
// Is the block already cached?
for(b = bcache.head.next; b != &bcache.head; b = b->next){
if(b->dev == dev && b->blockno == blockno){
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->blockno = blockno;
b->flags = B_BUSY;
release(&bcache.lock);
return b;
}
}
panic("bget: no buffers");
}
// Return a B_BUSY buf with the contents of the indicated block.
struct buf*
bread(uint dev, uint blockno)
{
struct buf *b;
b = bget(dev, blockno);
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);
}
//PAGEBREAK!
// Blank page.