>
> I've been debugging a hang in jbd2_journal_release_jbd_inode
> which is being seen on Power 6 systems quite a lot. When we get
> in the hung state, all I/O to the disk in question gets blocked
> where we stay indefinitely. Looking at the task list, I can see
> we are stuck in jbd2_journal_release_jbd_inode waiting on a
> wake up. I added some debug code to detect this scenario and
> dump additional data if we were stuck in jbd2_journal_release_jbd_inode
> for longer than 30 minutes. When it hit, I was able to see that
> i_flags was 0, suggesting we missed the wake up.
>
> This patch changes i_flags to be an unsigned long, uses bit operators
> to access it, and adds barriers around the accesses. Prior to applying
> this patch, we were regularly hitting this hang on numerous systems
> in our test environment. After applying the patch, the hangs no longer
> occur. Its still not clear to me why the j_list_lock doesn't protect us
> in this path.
Thanks for debugging this! I was thinking hard about how it could happen that
wake_up_bit doesn't wake up the waiter but I haven't found any explanation. All
the waitqueue work seems to be properly wrapped inside the j_list_lock so
even the waitqueue_active check in wake_up_bit should be fine.
I'd really like to understand what in my mind-model of spinlocks etc. is
wrong. So could you maybe run a test with the attached debug patch and
dump 'wait.seen' value in the hung task?
And one more question - if you remove 'waitqueue_active' check from
kernel/wait.c:__wake_up_bit
is the problem still present? Thanks a lot in advance.
Honza
--
Jan Kara <jack@xxxxxxx>
SuSE CR Labs
>From 7f305dbbe7cdb2ebfd410956f08ba4244ac578bd Mon Sep 17 00:00:00 2001
From: Jan Kara <jack@xxxxxxx>
Date: Wed, 21 Jul 2010 20:56:21 +0200
Subject: [PATCH] Debug bit wait queues in JBD2.
---
fs/jbd2/journal.c | 1 +
kernel/wait.c | 7 +++++--
2 files changed, 6 insertions(+), 2 deletions(-)
diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index 0368808..0ecd858 100644
--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -2211,6 +2211,7 @@ restart:
if (jinode->i_flags & JI_COMMIT_RUNNING) {
wait_queue_head_t *wq;
DEFINE_WAIT_BIT(wait, &jinode->i_flags, __JI_COMMIT_RUNNING);
+ wait.seen = 1;
wq = bit_waitqueue(&jinode->i_flags, __JI_COMMIT_RUNNING);
prepare_to_wait(wq, &wait.wait, TASK_UNINTERRUPTIBLE);
spin_unlock(&journal->j_list_lock);
diff --git a/kernel/wait.c b/kernel/wait.c
index c4bd3d8..46acfae 100644
--- a/kernel/wait.c
+++ b/kernel/wait.c
@@ -177,8 +177,11 @@ int wake_bit_function(wait_queue_t *wait, unsigned mode, int sync, void *arg)
= container_of(wait, struct wait_bit_queue, wait);
if (wait_bit->key.flags != key->flags ||
- wait_bit->key.bit_nr != key->bit_nr ||
- test_bit(key->bit_nr, key->flags))
+ wait_bit->key.bit_nr != key->bit_nr)
+ return 0;
+ if (wait_bit->seen == 1)
+ wait_bit->seen = 2 + test_bit(key->bit_nr, key->flags);
+ if (test_bit(key->bit_nr, key->flags))
return 0;
else
return autoremove_wake_function(wait, mode, sync, key);
--
1.6.4.2
