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 16:48:39 +0800
From:	Yongqiang Yang <xiaoqiangnk@...il.com>
To:	Jan Kara <jack@...e.cz>, Amir Goldstein <amir73il@...il.com>
Cc:	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, Mar 31, 2011 at 4:37 PM, Yongqiang Yang <xiaoqiangnk@...il.com> wrote:
> Hi everyone,
>
> Amir met a deadlock when he tested ext4 with snapshot.  The deadlock
> was reported on
> https://github.com/amir73il/ext4-snapshots/commit/56396185d922a73524a091b545e665543abf741a.
>  It is difficult to reproduce the deadlock.  There is a deadlock
> reported on http://www.spinics.net/lists/linux-ext4/msg23018.html.
> Actually, these two deadlocks come from a same source.
>
> Below are my analysis on the 1st one.  Mail is not a good place to
> describe parallel processes.  I have submitted the analysis to
> https://github.com/YANGYongqiang/ext4-snapshots/blob/9e0ae9ae9907125e6bf45aa91db296d4cc041b17/fs/ext4/BUGS#L143.
>  It is much more readable.
>
> -- deadlock in ext4 with snapshot
>       ext4 with snapshot calls freeze_super() to bring
>       a fs be in a clean state when a user takes a snapshot.
>
>     freeze                  truncate              kjournald
>
>                    |  ext4_ext_truncate     |
>    freeze_super()  |   starts a handle      |
>    sets s_frozen   |                        |
>                    |  ext4_ext_truncate     |
>                    |  holds i_data_sem      |
>  ext4_freeze()     |                        |   commit_transaction()
>   wait for updates |                        |   waits for i_data_sem
>                    |  ext4_free_blocks      |
>                    |  calls dquot_free_block|
>                    |                        |
>                    |  dquot_free_block call |
>                    |  ext4_dirty_inode      |
>                    |                        |
>                    |  ext4_dirty_inode      |
>                    |  trys to start a handle|
>                    |                        |
>                    |  block due to s_frozen |
>
> 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.

Virtually,  it is not right to block ext4_journal_start_sb() before we
confirm that current thread has no a active handle. But ext4 does like
that. Deadlock is thus easy to happen.   Right?

>
> Could someone explain why ext4 does like above but not follow ext3?
>
> Yongqiang.
> --
> Best Wishes
> Yongqiang Yang
>



-- 
Best Wishes
Yongqiang Yang
--
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