linux-ext4 - Re: RT/ext4/jbd2 circular dependency

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Date:	Mon, 27 Oct 2014 13:57:14 -0600
From:	Chris Friesen <chris.friesen@...driver.com>
To:	Austin Schuh <austin@...oton-tech.com>, <pavel@...linux.ru>
CC:	"J. Bruce Fields" <bfields@...ldses.org>,
	<linux-ext4@...r.kernel.org>, <tytso@....edu>,
	<adilger.kernel@...ger.ca>,
	rt-users <linux-rt-users@...r.kernel.org>
Subject: Re: RT/ext4/jbd2 circular dependency

On 10/23/2014 11:54 AM, Chris Friesen wrote:

> Basically it looks like we have a circular dependency involving the
> inode->i_data_sem rt_mutex, the PG_writeback bit, and the BJ_Shadow list.  It
> goes something like this:
>
> jbd2_journal_commit_transaction:
> 1) set page for writeback (set PG_writeback bit)
> 2) put jbd2 journal head on BJ_Shadow list
> 3) sleep on PG_writeback bit waiting for page writeback complete
>
> ext4_da_writepages:
> 1) ext4_map_blocks() acquires inode->i_data_sem for writing
> 2) do_get_write_access() sleeps waiting for jbd2 journal head to come off
> the BJ_Shadow list
>
> At this point the flush code can't run because it can't acquire
> inode->i_data_sem for reading, so the page will never get written out.
> Deadlock.

Just curious...would we expect lockdep to detect this sort of thing?  I 
wasn't sure if the introduction of the two wait queues would cause 
complications.

Chris
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