xv6/bio.c

// Buffer cache.
//
// The buffer cache is a linked list of buf structures holding
// cached copies of disk block contents.  Caching disk blocks
// in memory reduces the number of disk reads and also provides
// a synchronization point for disk blocks used by multiple processes.
//
// Interface:
// * To get a buffer for a particular disk block, call bread.
// * After changing buffer data, call bwrite to write it to disk.
// * When done with the buffer, call brelse.
// * Do not use the buffer after calling brelse.
// * Only one process at a time can use a buffer,
//     so do not keep them longer than necessary.
//
// The implementation uses two state flags internally:
// * B_VALID: the buffer data has been read from the disk.
// * B_DIRTY: the buffer data has been modified
//     and needs to be written to disk.

#include "types.h"
#include "defs.h"
#include "param.h"
#include "spinlock.h"
#include "sleeplock.h"
#include "fs.h"
#include "buf.h"

struct {
    struct spinlock lock;
    struct buf buf[NBUF];

    // Linked list of all buffers, through prev/next.
    // head.next is most recently used.
    struct buf head;
} bcache;

void binit(void) {
    struct buf *b;

    initlock(&bcache.lock, "bcache");

    // Create linked list of buffers
    bcache.head.prev = &bcache.head;
    bcache.head.next = &bcache.head;
    for (b = bcache.buf; b < bcache.buf + NBUF; b++) {
        b->next = bcache.head.next;
        b->prev = &bcache.head;
        initsleeplock(&b->lock, "buffer");
        bcache.head.next->prev = b;
        bcache.head.next = b;
    }
}

// Look through buffer cache for block on device dev.
// If not found, allocate a buffer.
// In either case, return locked buffer.

static struct buf* bget(uint dev, uint blockno) {
    struct buf *b;

    acquire(&bcache.lock);

    // Is the block already cached?
    for (b = bcache.head.next; b != &bcache.head; b = b->next) {
        if (b->dev == dev && b->blockno == blockno) {
            b->refcnt++;
            release(&bcache.lock);
            acquiresleep(&b->lock);
            return b;
        }
    }

    // Not cached; recycle an unused buffer.
    // Even if refcnt==0, B_DIRTY indicates a buffer is in use
    // because log.c has modified it but not yet committed it.

    for (b = bcache.head.prev; b != &bcache.head; b = b->prev) {
        if (b->refcnt == 0 && (b->flags & B_DIRTY) == 0) {
            b->dev = dev;
            b->blockno = blockno;
            b->flags = 0;
            b->refcnt = 1;
            release(&bcache.lock);
            acquiresleep(&b->lock);
            return b;
        }
    }
    panic("bget: no buffers");
}

// Return a locked buf with the contents of the indicated block.

struct buf*bread(uint dev, uint blockno) {
    struct buf *b;

    b = bget(dev, blockno);
    if ((b->flags & B_VALID) == 0) {
        iderw(b);
    }
    return b;
}

// Write b's contents to disk.  Must be locked.

void bwrite(struct buf *b) {
    if (!holdingsleep(&b->lock)) {
        panic("bwrite");
    }
    b->flags |= B_DIRTY;
    iderw(b);
}

// Release a locked buffer.
// Move to the head of the MRU list.

void brelse(struct buf *b) {
    if (!holdingsleep(&b->lock)) {
        panic("brelse");
    }

    releasesleep(&b->lock);

    acquire(&bcache.lock);
    b->refcnt--;
    if (b->refcnt == 0) {
        // no one is waiting for it.
        b->next->prev = b->prev;
        b->prev->next = b->next;
        b->next = bcache.head.next;
        b->prev = &bcache.head;
        bcache.head.next->prev = b;
        bcache.head.next = b;
    }

    release(&bcache.lock);
}