152 lines
3.4 KiB
C
152 lines
3.4 KiB
C
// Buffer cache.
|
|
//
|
|
// The buffer cache is a linked list of buf structures
|
|
// holding cached copies of disk block contents.
|
|
// Each buf has two state bits B_BUSY and B_VALID.
|
|
// If B_BUSY is set, it means that some code is currently
|
|
// editing buf, so other code is not allowed to look at it.
|
|
// To wait for a buffer that is B_BUSY, sleep on buf.
|
|
// (See bget below.)
|
|
//
|
|
// If B_VALID is set, it means that the memory contents
|
|
// have been initialized by reading them off the disk.
|
|
// (Conversely, if B_VALID is not set, the memory contents
|
|
// of buf must be initialized, often by calling bread,
|
|
// before being used.)
|
|
//
|
|
// After making changes to a buf's memory, call bwrite to flush
|
|
// the changes out to disk, to keep the disk and memory copies
|
|
// in sync.
|
|
//
|
|
// When finished with a buffer, call brelse to release the buffer
|
|
// (i.e., clear B_BUSY), so that others can access it.
|
|
//
|
|
// Bufs that are not B_BUSY are fair game for reuse for other
|
|
// disk blocks. It is not allowed to use a buf after calling brelse.
|
|
|
|
#include "types.h"
|
|
#include "param.h"
|
|
#include "x86.h"
|
|
#include "mmu.h"
|
|
#include "proc.h"
|
|
#include "defs.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)
|
|
break;
|
|
|
|
if(b != &bufhead){
|
|
if(b->flags & B_BUSY){
|
|
sleep(buf, &buf_table_lock);
|
|
goto loop;
|
|
}
|
|
b->flags |= B_BUSY;
|
|
// b->flags &= ~B_VALID; // Force reread from disk
|
|
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");
|
|
}
|
|
|
|
// Read buf's contents from disk.
|
|
struct buf*
|
|
bread(uint dev, uint sector)
|
|
{
|
|
struct buf *b;
|
|
|
|
b = bget(dev, sector);
|
|
if(b->flags & B_VALID)
|
|
return b;
|
|
|
|
ide_rw(dev & 0xff, sector, b->data, 1, 1);
|
|
b->flags |= B_VALID;
|
|
|
|
return b;
|
|
}
|
|
|
|
// Write buf's contents to disk.
|
|
// Must be locked.
|
|
void
|
|
bwrite(struct buf *b, uint sector)
|
|
{
|
|
if((b->flags & B_BUSY) == 0)
|
|
panic("bwrite");
|
|
|
|
ide_rw(b->dev & 0xff, sector, b->data, 1, 0);
|
|
b->flags |= B_VALID;
|
|
}
|
|
|
|
// 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);
|
|
}
|
|
|