linux-kernel - Re: next-20090310: ext4 hangs

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Date:	Wed, 25 Mar 2009 17:15:56 +0100
From:	Jan Kara <jack@...e.cz>
To:	Alexander Beregalov <a.beregalov@...il.com>
Cc:	Theodore Tso <tytso@....edu>,
	"linux-next@...r.kernel.org" <linux-next@...r.kernel.org>,
	linux-ext4@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: next-20090310: ext4 hangs

On Wed 25-03-09 18:29:10, Alexander Beregalov wrote:
> 2009/3/25 Jan Kara <jack@...e.cz>:
> > On Wed 25-03-09 18:18:43, Alexander Beregalov wrote:
> >> 2009/3/25 Jan Kara <jack@...e.cz>:
> >> >> > So, I think I need to try it on 2.6.29-rc7 again.
> >> >>   I've looked into this. Obviously, what's happenning is that we delete
> >> >> an inode and jbd2_journal_release_jbd_inode() finds inode is just under
> >> >> writeout in transaction commit and thus it waits. But it gets never woken
> >> >> up and because it has a handle from the transaction, every one eventually
> >> >> blocks on waiting for a transaction to finish.
> >> >>   But I don't really see how that can happen. The code is really
> >> >> straightforward and everything happens under j_list_lock... Strange.
> >> >  BTW: Is the system SMP?
> >> No, it is UP system.
> >  Even stranger. And do you have CONFIG_PREEMPT set?
> >
> >> The bug exists even in 2.6.29, I posted it with a new topic.
> >  OK, I've sort-of expected this.
> 
> CONFIG_PREEMPT_RCU=y
> CONFIG_PREEMPT_RCU_TRACE=y
> # CONFIG_PREEMPT_NONE is not set
> # CONFIG_PREEMPT_VOLUNTARY is not set
> CONFIG_PREEMPT=y
> CONFIG_DEBUG_PREEMPT=y
> # CONFIG_PREEMPT_TRACER is not set
> 
> config is attached.
  Thanks for the data. I still don't see how the wakeup can get lost. The
process even cannot be preempted when we are in the section protected by
j_list_lock... Can you send me a disassembly of functions
jbd2_journal_release_jbd_inode() and journal_submit_data_buffers() so that
I can see whether the compiler has not reordered something unexpectedly?
Thanks.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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