ext4: fix fast commit inode enqueueing during a full journal commit

When a full journal commit is on-going, any fast commit has to be enqueued into a different queue: FC_Q_STAGING instead of FC_Q_MAIN. This enqueueing is done only once, i.e. if an inode is already queued in a previous fast commit entry it won't be enqueued again. However, if a full commit starts _after_ the inode is enqueued into FC_Q_MAIN, the next fast commit needs to be done into FC_Q_STAGING. And this is not being done in function ext4_fc_track_template().

This patch fixes the issue by re-enqueuing an inode into the STAGING queue during the fast commit clean-up callback when doing a full commit. However, to prevent a race with a fast-commit, the clean-up callback has to be called with the journal locked.

This bug was found using fstest generic/047. This test creates several 32k bytes files, sync'ing each of them after it's creation, and then shutting down the filesystem. Some data may be loss in this operation; for example a file may have it's size truncated to zero.

Suggested-by Jan Kara <[email protected]>

Signed-off-by Luis Henriques (SUSE) <[email protected]>

---
Hi!

And here's another attempt to fix this bug. The most significant change is that now it doesn't assume a 'special' meaning for a tid of '0'. Which is a wrong assumption as Jan has shown.

I've also added a Suggested-by: tag, although Jan pretty much owns this patch -- I have simply tested it and sent it out!

 fs/ext4/fast_commit.c | 15 ++++++++++++++-
 fs/jbd2/journal.c     |  2 +-
 2 files changed, 15 insertions(+), 2 deletions(-)

fs/ext4/fast_commit.c +14 -1

index 3926a05eceee..df71fd5b1fed 100644
@@ -1288,8 +1288,21 @@ static void ext4_fc_cleanup(journal_t *journal, int full, tid_t tid)
 		list_del_init(&iter->i_fc_list);
 		ext4_clear_inode_state(&iter->vfs_inode,
 				       EXT4_STATE_FC_COMMITTING);
-		if (tid_geq(tid, iter->i_sync_tid))
+		if (tid_geq(tid, iter->i_sync_tid)) {
 			ext4_fc_reset_inode(&iter->vfs_inode);
+		} else if (full) {
+			/*
+			 * We are called after a full commit, inode has been
+			 * modified while the commit was running. Re-enqueue
+			 * the inode into STAGING, which will then be splice
+			 * back into MAIN. This cannot happen during
+			 * fastcommit because the journal is locked all the
+			 * time in that case (and tid doesn't increase so
+			 * tid check above isn't reliable).
+			 */
+			list_add_tail(&EXT4_I(&iter->vfs_inode)->i_fc_list,
+				      &sbi->s_fc_q[FC_Q_STAGING]);
+		}
 		/* Make sure EXT4_STATE_FC_COMMITTING bit is clear */
 		smp_mb();
 #if (BITS_PER_LONG < 64)

fs/jbd2/journal.c +1 -1

index 1ebf2393bfb7..291a431f8aaf 100644
@@ -740,9 +740,9 @@ EXPORT_SYMBOL(jbd2_fc_begin_commit);
  */
 static int __jbd2_fc_end_commit(journal_t *journal, tid_t tid, bool fallback)
 {
-	jbd2_journal_unlock_updates(journal);
 	if (journal->j_fc_cleanup_callback)
 		journal->j_fc_cleanup_callback(journal, 0, tid);
+	jbd2_journal_unlock_updates(journal);
 	write_lock(&journal->j_state_lock);
 	journal->j_flags &= ~JBD2_FAST_COMMIT_ONGOING;
 	if (fallback)

When a full journal commit is on-going, any fast commit has to be enqueued into a different queue: FC_Q_STAGING instead of FC_Q_MAIN. This enqueueing is done only once, i.e. if an inode is already queued in a previous fast commit entry it won't be enqueued again. However, if a full commit starts _after_ the inode is enqueued into FC_Q_MAIN, the next fast commit needs to be done into FC_Q_STAGING. And this is not being done in function ext4_fc_track_template().

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