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: Toshiyuki Okajima <toshi.okajima@jp.fujitsu.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
index a0e41a4c080e9b2d3a51f5b4c6404cb8b8bc9c2b..35ae096bed5dca819181c1d7bfa94194fa3b0c01 100644 (file)
--- a/fs/jbd2/transaction.c
+++ b/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();