linux-ext4 - Re: [PATCHv2 1/1] jbd2: Fix I/O hang in jbd2_journal_release_jbd

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Date:	Wed, 21 Jul 2010 21:02:52 +0200
From:	Jan Kara <jack@...e.cz>
To:	Brian King <brking@...ux.vnet.ibm.com>
Cc:	Josef Bacik <josef@...hat.com>, linux-ext4@...r.kernel.org,
	cmm@...ux.vnet.ibm.com, pmac@....ibm.com
Subject: Re: [PATCHv2 1/1] jbd2: Fix I/O hang in
	jbd2_journal_release_jbd_inode

> 
> 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@...e.cz>
SuSE CR Labs

View attachment "0001-Debug-bit-wait-queues-in-JBD2.patch" of type "text/x-diff" (1446 bytes)