--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/source/log.c	Sun Jan 06 19:27:03 2019 +0900
@@ -0,0 +1,186 @@
+#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;
+  memset(&log, 0, sizeof(log));
+  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.
+