xv6/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.
// Each buf has two state bits B_BUSY and B_VALID.
// If B_BUSY is set, it means that some code is currently
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// using buf, so other code is not allowed to use it.
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// To wait for a buffer that is B_BUSY, sleep on buf.
// (See bget below.)
//
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// If B_VALID is set, it means that the sector in b->data is
// the same as on the disk. If B_VALID is not set, the 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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//
// 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;
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// Linked list of all buffers, through prev/next.
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// bufhead->next is most recently used.
// bufhead->tail is least recently used.
struct buf bufhead;
void
binit(void)
{
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struct buf *b;
initlock(&buf_table_lock, "buf_table");
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// Create linked list of buffers
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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.
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// If not found, allocate fresh block.
// In either case, return locked buffer.
static struct buf*
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bget(uint dev, uint sector)
{
struct buf *b;
acquire(&buf_table_lock);
loop:
// Try for cached block.
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for(b = bufhead.next; b != &bufhead; b = b->next){
if((b->flags & (B_BUSY|B_VALID)) &&
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b->dev == dev && b->sector == sector){
if(b->flags & B_BUSY){
sleep(buf, &buf_table_lock);
goto loop;
}
b->flags |= B_BUSY;
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");
}
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// Return a B_BUSY buf with the contents of the indicated
// disk sector.
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struct buf*
bread(uint dev, uint sector)
{
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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b->flags &= ~B_WRITE;
ide_rw(b);
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b->flags |= B_VALID;
return b;
}
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// Write buf's contents to disk.
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// Must be locked.
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void
bwrite(struct buf *b)
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{
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if((b->flags & B_BUSY) == 0)
panic("bwrite");
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b->flags |= B_WRITE;
ide_rw(b);
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b->flags |= B_VALID;
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}
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// Release the buffer buf.
void
brelse(struct buf *b)
{
if((b->flags & B_BUSY) == 0)
panic("brelse");
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acquire(&buf_table_lock);
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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);
}