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jbd2: fix hung processes in jbd2_journal_lock_updates()
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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>
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Jan Kara authored and Theodore Ts'o committed Jan 5, 2012
1 parent 9b90e5e commit 9837d8e
Showing 1 changed file with 3 additions and 2 deletions.
5 changes: 3 additions & 2 deletions fs/jbd2/transaction.c
Original file line number Diff line number Diff line change
Expand Up @@ -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();
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