Skip to content

HTTPS clone URL

Subversion checkout URL

You can clone with HTTPS or Subversion.

Download ZIP
tree: 12abb1a561
Fetching contributors…

Cannot retrieve contributors at this time

186 lines (166 sloc) 4.545 kb
#include "types.h"
#include "defs.h"
#include "param.h"
#include "spinlock.h"
#include "fs.h"
#include "buf.h"
// Simple logging. Each system call that might write the file system
// should be surrounded with begin_trans() and commit_trans() calls.
//
// The log holds at most one transaction at a time. Commit forces
// the log (with commit record) to disk, then installs the affected
// blocks to disk, then erases the log. begin_trans() ensures that
// only one system call can be in a transaction; others must wait.
//
// Allowing only one transaction at a time means that the file
// system code doesn't have to worry about the possibility of
// one transaction reading a block that another one has modified,
// for example an i-node block.
//
// Read-only system calls don't need to use transactions, though
// this means that they may observe uncommitted data. I-node and
// buffer locks prevent read-only calls from seeing inconsistent data.
//
// The log is a physical re-do log containing disk blocks.
// The on-disk log format:
// header block, containing sector #s for block A, B, C, ...
// block A
// block B
// block C
// ...
// Log appends are synchronous.
// Contents of the header block, used for both the on-disk header block
// and to keep track in memory of logged sector #s before commit.
struct logheader {
int n;
int sector[LOGSIZE];
};
struct log {
struct spinlock lock;
int start;
int size;
int busy; // a transaction is active
int dev;
struct logheader lh;
};
struct log log;
static void recover_from_log(void);
void
initlog(void)
{
if (sizeof(struct logheader) >= BSIZE)
panic("initlog: too big logheader");
struct superblock sb;
initlock(&log.lock, "log");
readsb(ROOTDEV, &sb);
log.start = sb.size - sb.nlog;
log.size = sb.nlog;
log.dev = ROOTDEV;
recover_from_log();
}
// Copy committed blocks from log to their home location
static void
install_trans(void)
{
int tail;
for (tail = 0; tail < log.lh.n; tail++) {
struct buf *lbuf = bread(log.dev, log.start+tail+1); // read log block
struct buf *dbuf = bread(log.dev, log.lh.sector[tail]); // read dst
memmove(dbuf->data, lbuf->data, BSIZE); // copy block to dst
bwrite(dbuf); // write dst to disk
brelse(lbuf);
brelse(dbuf);
}
}
// Read the log header from disk into the in-memory log header
static void
read_head(void)
{
struct buf *buf = bread(log.dev, log.start);
struct logheader *lh = (struct logheader *) (buf->data);
int i;
log.lh.n = lh->n;
for (i = 0; i < log.lh.n; i++) {
log.lh.sector[i] = lh->sector[i];
}
brelse(buf);
}
// Write in-memory log header to disk.
// This is the true point at which the
// current transaction commits.
static void
write_head(void)
{
struct buf *buf = bread(log.dev, log.start);
struct logheader *hb = (struct logheader *) (buf->data);
int i;
hb->n = log.lh.n;
for (i = 0; i < log.lh.n; i++) {
hb->sector[i] = log.lh.sector[i];
}
bwrite(buf);
brelse(buf);
}
static void
recover_from_log(void)
{
read_head();
install_trans(); // if committed, copy from log to disk
log.lh.n = 0;
write_head(); // clear the log
}
void
begin_trans(void)
{
acquire(&log.lock);
while (log.busy) {
sleep(&log, &log.lock);
}
log.busy = 1;
release(&log.lock);
}
void
commit_trans(void)
{
if (log.lh.n > 0) {
write_head(); // Write header to disk -- the real commit
install_trans(); // Now install writes to home locations
log.lh.n = 0;
write_head(); // Erase the transaction from the log
}
acquire(&log.lock);
log.busy = 0;
wakeup(&log);
release(&log.lock);
}
// Caller has modified b->data and is done with the buffer.
// Append the block to the log and record the block number,
// but don't write the log header (which would commit the write).
// log_write() replaces bwrite(); a typical use is:
// bp = bread(...)
// modify bp->data[]
// log_write(bp)
// brelse(bp)
void
log_write(struct buf *b)
{
int i;
if (log.lh.n >= LOGSIZE || log.lh.n >= log.size - 1)
panic("too big a transaction");
if (!log.busy)
panic("write outside of trans");
for (i = 0; i < log.lh.n; i++) {
if (log.lh.sector[i] == b->sector) // log absorbtion?
break;
}
log.lh.sector[i] = b->sector;
struct buf *lbuf = bread(b->dev, log.start+i+1);
memmove(lbuf->data, b->data, BSIZE);
bwrite(lbuf);
brelse(lbuf);
if (i == log.lh.n)
log.lh.n++;
b->flags |= B_DIRTY; // XXX prevent eviction
}
//PAGEBREAK!
// Blank page.
Jump to Line
Something went wrong with that request. Please try again.