Dirt simple logging

Passes usertests and stressfs
Seems to recover correctly in a number of simple cases

log.c

@@ -0,0 +1,164 @@
+#include "types.h"
+#include "defs.h"
+#include "param.h"
+#include "mmu.h"
+#include "proc.h"
+#include "x86.h"
+#include "spinlock.h"
+#include "fs.h"
+#include "buf.h"
+
+// Dirt simple "logging" supporting only one transaction.  All file system calls
+// that potentially write a block should be wrapped in begin_trans and commit_trans,
+// so that there is never more than one transaction. This serializes all file system 
+// operations that potentially write, but simplifies recovery (only the last
+// one transaction to recover) and concurrency (don't have to worry about reading a modified
+// block from a transaction that hasn't committed yet).
+
+// The header of the log.  If head == 0, there are no log entries.  All entries till head
+// are committed. sector[] records the home sector for each block in the log 
+// (i.e., physical logging).
+struct logheader {
+  int head;   
+  int sector[LOGSIZE];
+};
+
+struct {
+  struct spinlock lock;
+  int start;
+  int size;
+  int intrans;
+  int dev;
+  struct logheader lh;
+} 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;
+
+  if (log.lh.head > 0)
+    cprintf("install_trans %d\n", log.lh.head);
+  for (tail = 0; tail < log.lh.head; tail++) {
+    cprintf("put entry %d to disk block %d\n", tail, log.lh.sector[tail]);
+    struct buf *lbuf = bread(log.dev, log.start+tail+1);   // read i'th block from log
+    struct buf *dbuf = bread(log.dev, log.lh.sector[tail]);  // read dst block
+    memmove(dbuf->data, lbuf->data, BSIZE);
+    bwrite(dbuf);
+    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.head = lh->head;
+  for (i = 0; i < log.lh.head; i++) {
+    log.lh.sector[i] = lh->sector[i];
+  }
+  brelse(buf);
+  if (log.lh.head > 0)
+    cprintf("read_head: %d\n", log.lh.head);
+}
+
+// Write the in-memory log header to disk, committing log entries till head
+static void
+write_head(void)
+{
+  if (log.lh.head > 0)
+    cprintf("write_head: %d\n", log.lh.head);
+
+  struct buf *buf = bread(log.dev, log.start);
+  struct logheader *hb = (struct logheader *) (buf->data);
+  int i;
+  hb->head = log.lh.head;
+  for (i = 0; i < log.lh.head; i++) {
+    hb->sector[i] = log.lh.sector[i];
+  }
+  bwrite(buf);
+  brelse(buf);
+}
+
+static void
+recover_from_log(void)
+{
+  read_head();      
+  install_trans();  // Install all transactions till head
+  log.lh.head = 0;
+  write_head();     //  Reclaim log
+}
+
+void
+begin_trans(void)
+{
+  acquire(&log.lock);
+  while (log.intrans) {
+    sleep(&log, &log.lock);
+  }
+  log.intrans = 1;
+  release(&log.lock);
+}
+
+void
+commit_trans(void)
+{
+  write_head();        // This causes all blocks till log.head to be commited
+  install_trans();     // Install all the transactions till head
+  log.lh.head = 0; 
+  write_head();        // Reclaim log
+
+  acquire(&log.lock);
+  log.intrans = 0;
+  wakeup(&log);
+  release(&log.lock);
+}
+
+// Write buffer into the log at log.head and record the block number log.lh.entry, but
+// don't write the log header (which would commit the write).
+void
+log_write(struct buf *b)
+{
+  int i;
+
+  if (log.lh.head >= LOGSIZE)
+    panic("too big a transaction");
+  if (!log.intrans)
+    panic("write outside of trans");
+
+  cprintf("log_write: %d %d\n", b->sector, log.lh.head);
+
+  for (i = 0; i < log.lh.head; 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.head)
+    log.lh.head++;
+}