[PATCH, RFC] jbd2: Call the commit callback before the transaction could get dropped

[Theodore Tso <tytso@mit.edu>]

This is what I plan to send to Linus to fix the problem.

commit 6fecbc3c7d27800e90a5f5fbca2fb2847e2c2854
Author: Theodore Ts'o <tytso@mit.edu>
Date:   Mon Oct 27 22:11:39 2008 -0400

    jbd2: Call the commit callback before the transaction could get dropped
    
    The transaction can potentially get dropped if there are no buffers
    that need to be written.  Make sure we call the commit callback before
    potentially deciding to drop the transaction.  Also avoid
    dereferencing the commit_transaction pointer in the marker for the
    same reason.
    
    This patch fixes the bug reported by Eric Paris at:
    http://bugzilla.kernel.org/show_bug.cgi?id=11838
    
    Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
index 8b119e1..ebc667b 100644
--- a/fs/jbd2/commit.c
+++ b/fs/jbd2/commit.c
@@ -974,6 +974,9 @@ restart_loop:
 	journal->j_committing_transaction = NULL;
 	spin_unlock(&journal->j_state_lock);
 
+	if (journal->j_commit_callback)
+		journal->j_commit_callback(journal, commit_transaction);
+
 	if (commit_transaction->t_checkpoint_list == NULL &&
 	    commit_transaction->t_checkpoint_io_list == NULL) {
 		__jbd2_journal_drop_transaction(journal, commit_transaction);
@@ -995,11 +998,8 @@ restart_loop:
 	}
 	spin_unlock(&journal->j_list_lock);
 
-	if (journal->j_commit_callback)
-		journal->j_commit_callback(journal, commit_transaction);
-
 	trace_mark(jbd2_end_commit, "dev %s transaction %d head %d",
-		   journal->j_devname, commit_transaction->t_tid,
+		   journal->j_devname, journal->j_commit_sequence,
 		   journal->j_tail_sequence);
 	jbd_debug(1, "JBD: commit %d complete, head %d\n",
 		  journal->j_commit_sequence, journal->j_tail_sequence);