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Date: Thu, 22 Dec 2011 20:56:50 +0900
From: Toshiyuki Okajima <toshi.okajima@...fujitsu.com>
To: tytso@....edu, adilger.kernel@...ger.ca
Cc: Yongqiang Yang <xiaoqiangnk@...il.com>, linux-ext4@...r.kernel.org
Subject: [PATCH 1/2] jbd2: jbd2_journal_stop needs an exclusive control to
synchronize around t_update operations
The following statements need an exclusive control for the critical code section
around t_update operations:
[jbd2_journal_stop()]
1445 /*
1446 * Once we drop t_updates, if it goes to zero the transaction
1447 * could start committing on us and eventually disappear. So
1448 * once we do this, we must not dereference transaction
1449 * pointer again.
1450 */
1451 tid = transaction->t_tid;
+ read_lock(&journal->j_state_lock);
----- critical code section ------------------------------------------------
1452 if (atomic_dec_and_test(&transaction->t_updates)) {
1453 wake_up(&journal->j_wait_updates);
1454 if (journal->j_barrier_count)
1455 wake_up(&journal->j_wait_transaction_locked);
1456 }
-----------------------------------------------------------------------------
+ read_unlock(&journal->j_state_lock);
1457
Because the functions which have the other critical code sections around t_update
operations,
- jbd2_journal_commit_transaction
- start_this_handle
- jbd2_journal_lock_updates
can not synchronize with jbd2_journal_stop.
ex) jbd2_journal_lock_updates
505 void jbd2_journal_lock_updates(journal_t *journal)
506 {
507 DEFINE_WAIT(wait);
508
509 write_lock(&journal->j_state_lock);
510 ++journal->j_barrier_count;
511
512 /* Wait until there are no running updates */
513 while (1) {
514 transaction_t *transaction = journal->j_running_transaction;
515
516 if (!transaction)
517 break;
518
519 spin_lock(&transaction->t_handle_lock);
----- critical code section ------------------------------------------------
520 if (!atomic_read(&transaction->t_updates)) {
521 spin_unlock(&transaction->t_handle_lock);
522 break;
523 }
524 prepare_to_wait(&journal->j_wait_updates, &wait,
525 TASK_UNINTERRUPTIBLE);
-----------------------------------------------------------------------------
526 spin_unlock(&transaction->t_handle_lock);
527 write_unlock(&journal->j_state_lock);
528 schedule();
529 finish_wait(&journal->j_wait_updates, &wait);
530 write_lock(&journal->j_state_lock);
531 }
532 write_unlock(&journal->j_state_lock);
Thefore, the following steps causes a hang-up of process1:
1) (process1) line 520 in jbd2_journal_lock_updates
transaction->t_updates is equal to 1, and then goto 4).
2) (process2) line 1452 in jbd2_journal_stop
transaction->t_updates becomes to 0, and then goto 3).
3) (process2) line 1453 in jbd2_journal_stop
wake_up(&journal->j_wait_updates) tries to wake someone up.
4) (process1) line 524 in jbd2_journal_lock_updates
prepare to sleep itself, and then goto 5).
5) (process1) line 528 in jbd2_journal_lock_updates
sleep forever because process2 doesn't wake it up anymore.
Similar problem also exists for j_barrier_count operations but it can be
fixed, too:
[jbd2_journal_lock_updates]
505 void jbd2_journal_lock_updates(journal_t *journal)
506 {
507 DEFINE_WAIT(wait);
508
509 write_lock(&journal->j_state_lock);
----------------------------------------------------------------------------
510 ++journal->j_barrier_count;
----------------------------------------------------------------------------
...
532 write_unlock(&journal->j_state_lock);
[jbd2_journal_stop]
1445 /*
1446 * Once we drop t_updates, if it goes to zero the transaction
1447 * could start committing on us and eventually disappear. So
1448 * once we do this, we must not dereference transaction
1449 * pointer again.
1450 */
1451 tid = transaction->t_tid;
+ read_lock(&journal->j_state_lock);
1452 if (atomic_dec_and_test(&transaction->t_updates)) {
1453 wake_up(&journal->j_wait_updates);
----------------------------------------------------------------------------
1454 if (journal->j_barrier_count)
1455 wake_up(&journal->j_wait_transaction_locked);
----------------------------------------------------------------------------
1456 }
+ read_unlock(&journal->j_state_lock);
1457
Signed-off-by: Toshiyuki Okajima <toshi.okajima@...fujitsu.com>
---
fs/jbd2/transaction.c | 2 ++
1 files changed, 2 insertions(+), 0 deletions(-)
diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
index a0e41a4..76f2eca 100644
--- a/fs/jbd2/transaction.c
+++ b/fs/jbd2/transaction.c
@@ -1449,11 +1449,13 @@ int jbd2_journal_stop(handle_t *handle)
* pointer again.
*/
tid = transaction->t_tid;
+ read_lock(&journal->j_state_lock);
if (atomic_dec_and_test(&transaction->t_updates)) {
wake_up(&journal->j_wait_updates);
if (journal->j_barrier_count)
wake_up(&journal->j_wait_transaction_locked);
}
+ read_unlock(&journal->j_state_lock);
if (wait_for_commit)
err = jbd2_log_wait_commit(journal, tid);
--
1.5.5.6
--
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