linux-ext4 - Re: [BUG] ext3: cannot unfreeze a filesystem due to a deadlock

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Date:	Wed, 7 Sep 2011 13:56:08 -0400
From:	Greg Freemyer <greg.freemyer@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Masayoshi MIZUMA <m.mizuma@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Valerie Aurora <val@...consulting.com>
Subject: Re: [BUG] ext3: cannot unfreeze a filesystem due to a deadlock

On Wed, Sep 7, 2011 at 1:34 PM, Jan Kara <jack@...e.cz> wrote:
>  Hello,
>
>  Thanks for report!
>
> On Wed 07-09-11 12:29:30, Masayoshi MIZUMA wrote:
>> When I checked the freeze feature for ext3 filesystem using fsfreeze
>> command at 3.1.0-rc4, I think the following deadlock problem happened.
>>
>> How to reproduce:
>>  # mkfs -t ext3 /dev/sdd1
>>  # mount /dev/sdd1 /MNT
>>  # ./fsstress -d /MNT/tmp -n 10 -p 1000 > /dev/null 2>&1 &
>>  # fsfreeze -f /MNT
>>  # fsfreeze -u /MNT
>>
>>  If this deadlock is reproduced, "fsfreeze -u /MNT" does not return.
>>
>> The detail of deadlock:
>> o [flush-8:16:1523]
>>   wb_do_writeback
>>    wb_writeback
>>    ...
>>      ext3_journalled_writepage
>>       journal_start
>>        start_this_handle
>>        # waiting until journal->j_barrier_count turns 0...
>>        # j_barrier_count was incremented by journal_lock_updates()
>>        # via ext3_freeze().
>>
>> o [fsstress:2673]
>>   sys_sync
>>    sync_filesystems
>>     iterate_supers
>>      down_read(sb->s_umount)
>>      sync_one_sb
>>       __sync_filesystem
>>        writeback_inodes_sb
>>         writeback_inodes_sb_nr
>>          wait_for_completion
>>           wait_for_common
>>           # waiting for completion of [flush-8:16:1523]...
>>
>> o [fsfreeze:2749]
>>   sys_ioctl
>>    do_vfs_ioctl
>>     thaw_super
>>     # waiting for down_write(sb->s_umount)...
>>     # [fsfreeze:2673] did down_read(sb->s_umount).
>  Yes, this is a classical deadlock that can happen for any filesystem. The
> problem is flusher thread holds s_umount semaphore (either directly, or as
> in your case, indirectly via blocked sync) and tries to do some IO which
> blocks on frozen filesystem. It's particularly easy to hit for ext3 because
> it doesn't do vfs_check_frozen() checks but all other filesystems have the
> race window as well. Val Henson is working on fixing the problem - she even
> has some first version of patches I believe.
>
>                                                                Honza

xfstests test 068 has been around since kernel 2.4 days and should
have caught it if xfs is impacted.

I know I ran the 2002 version many times to prove to myself that
fsfreeze for xfs was stable when teamed with LVM.  (It wasn't when I
first wrote 068 way back then).

068 has been greatly simplified since 2002, but it still looks like it
should do a good job.

Is there a problem with 068?  Does it need extra test coverage even for xfs?

Greg
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