2006-09-07 16:28:12 +02:00
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// Buffer cache.
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//
|
2007-08-27 16:09:30 +02:00
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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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2006-09-07 16:28:12 +02:00
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//
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2007-08-27 16:09:30 +02:00
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// Interface:
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// * To get a buffer for a particular disk block, call bread.
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2011-10-11 12:41:37 +02:00
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// * After changing buffer data, call bwrite to write it to disk.
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2007-08-27 16:09:30 +02:00
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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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2006-09-07 16:28:12 +02:00
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//
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2011-08-29 23:18:40 +02:00
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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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2011-10-11 12:41:37 +02:00
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// * B_VALID: the buffer data has been read from the disk.
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2007-08-27 16:09:30 +02:00
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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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2006-09-07 16:28:12 +02:00
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2006-07-21 15:18:04 +02:00
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#include "types.h"
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#include "defs.h"
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2007-08-28 01:26:33 +02:00
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#include "param.h"
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2006-07-21 15:18:04 +02:00
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#include "spinlock.h"
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#include "buf.h"
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2009-05-31 03:29:17 +02:00
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struct {
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struct spinlock lock;
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struct buf buf[NBUF];
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2006-08-11 00:08:14 +02:00
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2009-05-31 03:29:17 +02:00
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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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2006-08-13 00:34:13 +02:00
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2006-08-11 00:08:14 +02:00
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void
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binit(void)
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{
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2006-08-13 00:34:13 +02:00
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struct buf *b;
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2009-05-31 07:12:21 +02:00
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initlock(&bcache.lock, "bcache");
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2006-08-13 00:34:13 +02:00
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|
2007-08-28 07:00:53 +02:00
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//PAGEBREAK!
|
2006-09-07 16:28:12 +02:00
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// Create linked list of buffers
|
2009-05-31 03:29:17 +02:00
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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;
|
2006-08-13 00:34:13 +02:00
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|
}
|
2006-08-11 00:08:14 +02:00
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|
|
}
|
2006-07-21 15:18:04 +02:00
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|
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|
2007-08-08 11:50:26 +02:00
|
|
|
// Look through buffer cache for sector on device dev.
|
2006-09-07 16:28:12 +02:00
|
|
|
// If not found, allocate fresh block.
|
2011-10-11 12:41:37 +02:00
|
|
|
// In either case, return B_BUSY buffer.
|
2006-09-07 16:28:12 +02:00
|
|
|
static struct buf*
|
2006-09-07 16:39:05 +02:00
|
|
|
bget(uint dev, uint sector)
|
2006-07-21 15:18:04 +02:00
|
|
|
{
|
2006-08-12 13:38:57 +02:00
|
|
|
struct buf *b;
|
2006-07-21 15:18:04 +02:00
|
|
|
|
2009-05-31 03:29:17 +02:00
|
|
|
acquire(&bcache.lock);
|
2011-08-29 23:18:40 +02:00
|
|
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|
|
loop:
|
2011-10-11 12:41:37 +02:00
|
|
|
// Is the sector already cached?
|
2009-05-31 03:29:17 +02:00
|
|
|
for(b = bcache.head.next; b != &bcache.head; b = b->next){
|
|
|
|
if(b->dev == dev && b->sector == sector){
|
2011-08-29 23:18:40 +02:00
|
|
|
if(!(b->flags & B_BUSY)){
|
|
|
|
b->flags |= B_BUSY;
|
|
|
|
release(&bcache.lock);
|
|
|
|
return b;
|
|
|
|
}
|
|
|
|
sleep(b, &bcache.lock);
|
|
|
|
goto loop;
|
2007-08-08 11:50:26 +02:00
|
|
|
}
|
|
|
|
}
|
|
|
|
|
2011-10-14 16:23:23 +02:00
|
|
|
// Not cached; recycle some non-busy and clean buffer.
|
2009-05-31 03:29:17 +02:00
|
|
|
for(b = bcache.head.prev; b != &bcache.head; b = b->prev){
|
2011-10-14 16:23:23 +02:00
|
|
|
if((b->flags & B_BUSY) == 0 && (b->flags & B_DIRTY) == 0){
|
2007-08-08 11:50:26 +02:00
|
|
|
b->dev = dev;
|
|
|
|
b->sector = sector;
|
2011-08-29 23:18:40 +02:00
|
|
|
b->flags = B_BUSY;
|
|
|
|
release(&bcache.lock);
|
2007-08-08 11:50:26 +02:00
|
|
|
return b;
|
2006-07-21 15:18:04 +02:00
|
|
|
}
|
|
|
|
}
|
2007-08-08 11:50:26 +02:00
|
|
|
panic("bget: no buffers");
|
2006-07-21 15:18:04 +02:00
|
|
|
}
|
|
|
|
|
2011-08-29 23:18:40 +02:00
|
|
|
// Return a B_BUSY buf with the contents of the indicated disk sector.
|
2006-09-06 19:27:19 +02:00
|
|
|
struct buf*
|
2006-07-21 15:18:04 +02:00
|
|
|
bread(uint dev, uint sector)
|
|
|
|
{
|
|
|
|
struct buf *b;
|
|
|
|
|
2006-09-07 16:28:12 +02:00
|
|
|
b = bget(dev, sector);
|
2007-08-27 16:09:30 +02:00
|
|
|
if(!(b->flags & B_VALID))
|
2009-03-08 23:07:13 +01:00
|
|
|
iderw(b);
|
2006-07-21 15:18:04 +02:00
|
|
|
return b;
|
|
|
|
}
|
|
|
|
|
2011-10-11 12:41:37 +02:00
|
|
|
// Write b's contents to disk. Must be B_BUSY.
|
2006-08-07 03:38:46 +02:00
|
|
|
void
|
2007-08-22 08:01:32 +02:00
|
|
|
bwrite(struct buf *b)
|
2006-08-07 03:38:46 +02:00
|
|
|
{
|
2011-08-29 23:18:40 +02:00
|
|
|
if((b->flags & B_BUSY) == 0)
|
2006-09-07 17:29:54 +02:00
|
|
|
panic("bwrite");
|
2007-08-27 16:09:30 +02:00
|
|
|
b->flags |= B_DIRTY;
|
2009-03-08 23:07:13 +01:00
|
|
|
iderw(b);
|
2006-08-07 03:38:46 +02:00
|
|
|
}
|
|
|
|
|
2011-10-11 12:41:37 +02:00
|
|
|
// Release a B_BUSY buffer.
|
|
|
|
// Move to the head of the MRU list.
|
2006-07-21 15:18:04 +02:00
|
|
|
void
|
|
|
|
brelse(struct buf *b)
|
|
|
|
{
|
2011-08-29 23:18:40 +02:00
|
|
|
if((b->flags & B_BUSY) == 0)
|
2006-07-21 15:18:04 +02:00
|
|
|
panic("brelse");
|
2006-09-06 19:27:19 +02:00
|
|
|
|
2009-05-31 03:29:17 +02:00
|
|
|
acquire(&bcache.lock);
|
2011-08-29 23:18:40 +02:00
|
|
|
|
2006-08-13 00:34:13 +02:00
|
|
|
b->next->prev = b->prev;
|
|
|
|
b->prev->next = b->next;
|
2009-05-31 03:29:17 +02:00
|
|
|
b->next = bcache.head.next;
|
|
|
|
b->prev = &bcache.head;
|
|
|
|
bcache.head.next->prev = b;
|
|
|
|
bcache.head.next = b;
|
2006-08-13 00:34:13 +02:00
|
|
|
|
2011-08-29 23:18:40 +02:00
|
|
|
b->flags &= ~B_BUSY;
|
|
|
|
wakeup(b);
|
2006-07-21 15:18:04 +02:00
|
|
|
|
2009-05-31 03:29:17 +02:00
|
|
|
release(&bcache.lock);
|
2006-07-21 15:18:04 +02:00
|
|
|
}
|
|
|
|
|