UBIFS: synchronize write-buffer before switching to the next bud

Currently when UBIFS fills up the current bud (which is the last in the journal
head) and switches to the next bud, it first writes the log reference node for
the next bud and only after this synchronizes the write-buffer of the previous
bud. This is not a big deal, but an unclean power cut may lead to a situation
when we have corruption in a next-to-last bud, although it is much more logical
that we have to have corruption only in the last bud.

This patch also removes write-buffer synchronization from
'ubifs_wbuf_seek_nolock()' because this is not needed anymore (we synchronize
the write-buffer explicitly everywhere now) and also because this is just
prone to various errors.
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

fs/ubifs/gc.c

@@ -100,6 +100,10 @@ static int switch_gc_head(struct ubifs_info *c)
 	if (err)
 		return err;
 
+	err = ubifs_wbuf_sync_nolock(wbuf);
+	if (err)
+		return err;
+
 	err = ubifs_add_bud_to_log(c, GCHD, gc_lnum, 0);
 	if (err)
 		return err;

fs/ubifs/io.c

@@ -452,8 +452,8 @@ int ubifs_wbuf_sync_nolock(struct ubifs_wbuf *wbuf)
  * @dtype: data type
  *
  * This function targets the write-buffer to logical eraseblock @lnum:@offs.
- * The write-buffer is synchronized if it is not empty. Returns zero in case of
- * success and a negative error code in case of failure.
+ * The write-buffer has to be empty. Returns zero in case of success and a
+ * negative error code in case of failure.
  */
 int ubifs_wbuf_seek_nolock(struct ubifs_wbuf *wbuf, int lnum, int offs,
 			   int dtype)
@@ -465,13 +465,7 @@ int ubifs_wbuf_seek_nolock(struct ubifs_wbuf *wbuf, int lnum, int offs,
 	ubifs_assert(offs >= 0 && offs <= c->leb_size);
 	ubifs_assert(offs % c->min_io_size == 0 && !(offs & 7));
 	ubifs_assert(lnum != wbuf->lnum);
-
-	if (wbuf->used > 0) {
-		int err = ubifs_wbuf_sync_nolock(wbuf);
-
-		if (err)
-			return err;
-	}
+	ubifs_assert(wbuf->used == 0);
 
 	spin_lock(&wbuf->lock);
 	wbuf->lnum = lnum;

fs/ubifs/journal.c

@@ -141,14 +141,8 @@ static int reserve_space(struct ubifs_info *c, int jhead, int len)
 	 * LEB with some empty space.
 	 */
 	lnum = ubifs_find_free_space(c, len, &offs, squeeze);
-	if (lnum >= 0) {
-		/* Found an LEB, add it to the journal head */
-		err = ubifs_add_bud_to_log(c, jhead, lnum, offs);
-		if (err)
-			goto out_return;
-		/* A new bud was successfully allocated and added to the log */
+	if (lnum >= 0)
 		goto out;
-	}
 
 	err = lnum;
 	if (err != -ENOSPC)
@@ -203,12 +197,23 @@ static int reserve_space(struct ubifs_info *c, int jhead, int len)
 		return 0;
 	}
 
-	err = ubifs_add_bud_to_log(c, jhead, lnum, 0);
-	if (err)
-		goto out_return;
 	offs = 0;
 
 out:
+	/*
+	 * Make sure we synchronize the write-buffer before we add the new bud
+	 * to the log. Otherwise we may have a power cut after the log
+	 * reference node for the last bud (@lnum) is written but before the
+	 * write-buffer data are written to the next-to-last bud
+	 * (@wbuf->lnum). And the effect would be that the recovery would see
+	 * that there is corruption in the next-to-last bud.
+	 */
+	err = ubifs_wbuf_sync_nolock(wbuf);
+	if (err)
+		goto out_return;
+	err = ubifs_add_bud_to_log(c, jhead, lnum, offs);
+	if (err)
+		goto out_return;
 	err = ubifs_wbuf_seek_nolock(wbuf, lnum, offs, wbuf->dtype);
 	if (err)
 		goto out_unlock;