linux-ext4 - Re: [RFC][PATCH] Re: [BUG] ext4: cannot unfreeze a filesystem due to a deadlock

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Date:	Thu, 31 Mar 2011 18:28:35 +0200
From:	Jan Kara <jack@...e.cz>
To:	Yongqiang Yang <xiaoqiangnk@...il.com>
Cc:	Eric Sandeen <sandeen@...hat.com>, Jan Kara <jack@...e.cz>,
	Amir Goldstein <amir73il@...il.com>,
	Toshiyuki Okajima <toshi.okajima@...fujitsu.com>,
	Ted Ts'o <tytso@....edu>,
	Masayoshi MIZUMA <m.mizuma@...fujitsu.com>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [RFC][PATCH] Re: [BUG] ext4: cannot unfreeze a filesystem due
 to a deadlock

On Thu 31-03-11 22:36:46, Yongqiang Yang wrote:
> On Thu, Mar 31, 2011 at 10:04 PM, Eric Sandeen <sandeen@...hat.com> wrote:
> > On 3/31/11 3:37 AM, Yongqiang Yang wrote:
> >
> >> in ext3, ext3_freeze() prevents journal from being updated by
> >> lock_journal_updates(), ext3_unfreeze() allow journal to be updated by
> >> unlock_journal_updates().
> >>
> >> in ext4, however, before ext4_freeze() returns, it unlock journal, and
> >> ext4 prevents journal from being updated by s_frozen. s_frozen is in
> >> an upper layer, so it is out control of ext4 and deadlock is easy to
> >> happen.
> >>
> >> Could someone explain why ext4 does like above but not follow ext3?
> >>
> >> Yongqiang.
> >
> > That was me, I think ...
> 
> Thank you, Eric.
> 
> I think ext4_journal_start() should check if current thread has an
> active handle before  vfs_check_frozen(), if so, current handle will
> be returned. Thus, we can avoid deadlocks.
> 
> Do you agree with me?  If I am right, I will send a patch.
  Yes, definitely. This was exactly what I wanted to propose as well.
Thanks for looking into this.

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