jbd2: fix hung processes in jbd2_journal_lock_updates()

Toshiyuki Okajima found out that when running

for ((i=0; i < 100000; i++)); do
        if ((i%2 == 0)); then
                chattr +j /mnt/file
        else
                chattr -j /mnt/file
        fi
        echo "0" >> /mnt/file
done

process sometimes hangs indefinitely in jbd2_journal_lock_updates().

Toshiyuki identified that the following race happens:

jbd2_journal_lock_updates()            |jbd2_journal_stop()
---------------------------------------+---------------------------------------
 write_lock(&journal->j_state_lock)    |    .
 ++journal->j_barrier_count            |    .
 spin_lock(&tran->t_handle_lock)       |    .
 atomic_read(&tran->t_updates) //not 0 |
                                       | atomic_dec_and_test(&tran->t_updates)
                                       |    // t_updates = 0
                                       | wake_up(&journal->j_wait_updates)
 prepare_to_wait()                     |    // no process is woken up.
 spin_unlock(&tran->t_handle_lock)     |
 write_unlock(&journal->j_state_lock)  |
 schedule() // never return            |

We fix the problem by first calling prepare_to_wait() and only after that
checking t_updates in jbd2_journal_lock_updates().
Reported-and-analyzed-by: NToshiyuki Okajima <toshi.okajima@jp.fujitsu.com>
Signed-off-by: NJan Kara <jack@suse.cz>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/jbd2/transaction.c

@@ -517,12 +517,13 @@ void jbd2_journal_lock_updates(journal_t *journal)
 			break;
 
 		spin_lock(&transaction->t_handle_lock);
+		prepare_to_wait(&journal->j_wait_updates, &wait,
+				TASK_UNINTERRUPTIBLE);
 		if (!atomic_read(&transaction->t_updates)) {
 			spin_unlock(&transaction->t_handle_lock);
+			finish_wait(&journal->j_wait_updates, &wait);
 			break;
 		}
-		prepare_to_wait(&journal->j_wait_updates, &wait,
-				TASK_UNINTERRUPTIBLE);
 		spin_unlock(&transaction->t_handle_lock);
 		write_unlock(&journal->j_state_lock);
 		schedule();