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 09:04:13 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Yongqiang Yang <xiaoqiangnk@...il.com>
CC:	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 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 ...

commit 6b0310fbf087ad6e9e3b8392adca97cd77184084
Author: Eric Sandeen <sandeen@...hat.com>
Date:   Sun May 16 02:00:00 2010 -0400

    ext4: don't return to userspace after freezing the fs with a mutex held

    ext4_freeze() used jbd2_journal_lock_updates() which takes
    the j_barrier mutex, and then returns to userspace.  The
    kernel does not like this:

    ================================================
    [ BUG: lock held when returning to user space! ]
    ------------------------------------------------
    lvcreate/1075 is leaving the kernel with locks still held!
    1 lock held by lvcreate/1075:
     #0:  (&journal->j_barrier){+.+...}, at: [<ffffffff811c6214>]
    jbd2_journal_lock_updates+0xe1/0xf0

    Use vfs_check_frozen() added to ext4_journal_start_sb() and
    ext4_force_commit() instead.

    Addresses-Red-Hat-Bugzilla: #568503


    Signed-off-by: Eric Sandeen <sandeen@...hat.com>
    Signed-off-by: "Theodore Ts'o" <tytso@....edu>
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