jbd2: optimize jbd2_log_do_checkpoint() a bit

When we discover written out buffer in transaction checkpoint list we
don't have to recheck validity of a transaction. Either this is the
last buffer in a transaction - and then we are done - or this isn't
and then we can just take another buffer from the checkpoint list
without dropping j_list_lock.
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/jbd2/checkpoint.c

@@ -285,9 +285,10 @@ int jbd2_log_do_checkpoint(journal_t *journal)
 			if (unlikely(buffer_write_io_error(bh)) && !result)
 				result = -EIO;
 			BUFFER_TRACE(bh, "remove from checkpoint");
-			__jbd2_journal_remove_checkpoint(jh);
-			spin_unlock(&journal->j_list_lock);
-			goto retry;
+			if (__jbd2_journal_remove_checkpoint(jh))
+				/* The transaction was released; we're done */
+				goto out;
+			continue;
 		}
 		/*
 		 * Important: we are about to write the buffer, and