140 lines
3.2 KiB
C
140 lines
3.2 KiB
C
// Buffer cache.
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//
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// The buffer cache is a linked list of buf structures holding
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// cached copies of disk block contents. Caching disk blocks
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// in memory reduces the number of disk reads and also provides
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// a synchronization point for disk blocks used by multiple processes.
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//
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// Interface:
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// * To get a buffer for a particular disk block, call bread.
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// * After changing buffer data, call bwrite to write it to disk.
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// * When done with the buffer, call brelse.
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// * Do not use the buffer after calling brelse.
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// * Only one process at a time can use a buffer,
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// so do not keep them longer than necessary.
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//
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// The implementation uses three state flags internally:
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// * B_BUSY: the block has been returned from bread
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// and has not been passed back to brelse.
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// * B_VALID: the buffer data has been read from the disk.
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// * B_DIRTY: the buffer data has been modified
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// and needs to be written to disk.
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#include "types.h"
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#include "defs.h"
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#include "param.h"
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#include "spinlock.h"
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#include "buf.h"
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struct {
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struct spinlock lock;
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struct buf buf[NBUF];
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// Linked list of all buffers, through prev/next.
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// head.next is most recently used.
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struct buf head;
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} bcache;
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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(&bcache.lock, "bcache");
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//PAGEBREAK!
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// Create linked list of buffers
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bcache.head.prev = &bcache.head;
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bcache.head.next = &bcache.head;
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for(b = bcache.buf; b < bcache.buf+NBUF; b++){
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b->next = bcache.head.next;
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b->prev = &bcache.head;
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b->dev = -1;
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bcache.head.next->prev = b;
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bcache.head.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 B_BUSY 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(&bcache.lock);
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loop:
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// Is the sector already cached?
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for(b = bcache.head.next; b != &bcache.head; b = b->next){
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if(b->dev == dev && b->sector == sector){
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if(!(b->flags & B_BUSY)){
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b->flags |= B_BUSY;
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release(&bcache.lock);
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return b;
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}
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sleep(b, &bcache.lock);
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goto loop;
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}
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}
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// Not cached; recycle some non-busy and clean buffer.
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for(b = bcache.head.prev; b != &bcache.head; b = b->prev){
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if((b->flags & B_BUSY) == 0 && (b->flags & B_DIRTY) == 0){
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b->dev = dev;
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b->sector = sector;
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b->flags = B_BUSY;
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release(&bcache.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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// Return a B_BUSY buf with the contents of the indicated disk sector.
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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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iderw(b);
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return b;
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}
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// Write b's contents to disk. Must be B_BUSY.
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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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b->flags |= B_DIRTY;
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iderw(b);
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}
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// Release a B_BUSY buffer.
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// Move to the head of the MRU list.
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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(&bcache.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 = bcache.head.next;
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b->prev = &bcache.head;
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bcache.head.next->prev = b;
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bcache.head.next = b;
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b->flags &= ~B_BUSY;
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wakeup(b);
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release(&bcache.lock);
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}
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