150 lines
3.3 KiB
C
150 lines
3.3 KiB
C
// Buffer cache.
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//
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// The buffer cache is a linked list of buf structures
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// holding cached copies of disk block contents.
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// Each buf has two state bits B_BUSY and B_VALID.
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// If B_BUSY is set, it means that some code is currently
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// editing buf, so other code is not allowed to look at it.
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// To wait for a buffer that is B_BUSY, sleep on buf.
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// (See bget below.)
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//
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// If B_VALID is set, it means that the memory contents
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// have been initialized by reading them off the disk.
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// (Conversely, if B_VALID is not set, the memory contents
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// of buf must be initialized, often by calling bread,
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// before being used.)
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//
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// After making changes to a buf's memory, call bwrite to flush
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// the changes out to disk, to keep the disk and memory copies
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// in sync.
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//
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// When finished with a buffer, call brelse to release the buffer
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// (i.e., clear B_BUSY), so that others can access it.
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//
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// Bufs that are not B_BUSY are fair game for reuse for other
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// disk blocks. It is not allowed to use a buf after calling brelse.
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#include "types.h"
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#include "param.h"
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#include "x86.h"
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#include "mmu.h"
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#include "proc.h"
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#include "defs.h"
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#include "spinlock.h"
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#include "buf.h"
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struct buf buf[NBUF];
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struct spinlock buf_table_lock;
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// Linked list of all buffers, through prev/next.
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// bufhead->next is most recently used.
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// bufhead->tail is least recently used.
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struct buf bufhead;
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void
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binit(void)
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{
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struct buf *b;
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initlock(&buf_table_lock, "buf_table");
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// Create linked list of buffers
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bufhead.prev = &bufhead;
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bufhead.next = &bufhead;
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for(b = buf; b < buf+NBUF; b++){
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b->next = bufhead.next;
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b->prev = &bufhead;
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bufhead.next->prev = b;
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bufhead.next = b;
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}
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}
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// Look through buffer cache for sector on device dev.
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// If not found, allocate fresh block.
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// In either case, return locked buffer.
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static struct buf*
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bget(uint dev, uint sector)
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{
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struct buf *b;
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acquire(&buf_table_lock);
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loop:
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// Try for cached block.
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for(b = bufhead.next; b != &bufhead; b = b->next){
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if((b->flags & (B_BUSY|B_VALID)) &&
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b->dev == dev && b->sector == sector){
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if(b->flags & B_BUSY){
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sleep(buf, &buf_table_lock);
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goto loop;
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}
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b->flags |= B_BUSY;
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// b->flags &= ~B_VALID; // Force reread from disk
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release(&buf_table_lock);
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return b;
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}
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}
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// Allocate fresh block.
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for(b = bufhead.prev; b != &bufhead; b = b->prev){
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if((b->flags & B_BUSY) == 0){
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b->flags = B_BUSY;
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b->dev = dev;
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b->sector = sector;
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release(&buf_table_lock);
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return b;
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}
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}
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panic("bget: no buffers");
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}
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// Read buf's contents from disk.
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struct buf*
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bread(uint dev, uint sector)
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{
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struct buf *b;
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b = bget(dev, sector);
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if(b->flags & B_VALID)
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return b;
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ide_rw(dev & 0xff, sector, b->data, 1, 1);
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b->flags |= B_VALID;
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return b;
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}
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// Write buf's contents to disk.
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// Must be locked.
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void
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bwrite(struct buf *b)
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{
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if((b->flags & B_BUSY) == 0)
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panic("bwrite");
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ide_rw(b->dev & 0xff, b->sector, b->data, 1, 0);
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b->flags |= B_VALID;
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}
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// Release the buffer buf.
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void
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brelse(struct buf *b)
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{
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if((b->flags & B_BUSY) == 0)
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panic("brelse");
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acquire(&buf_table_lock);
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b->next->prev = b->prev;
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b->prev->next = b->next;
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b->next = bufhead.next;
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b->prev = &bufhead;
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bufhead.next->prev = b;
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bufhead.next = b;
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b->flags &= ~B_BUSY;
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wakeup(buf);
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release(&buf_table_lock);
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}
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