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

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Date:	Thu, 30 Oct 2014 22:11:26 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Theodore Ts'o <tytso@....edu>
cc:	Chris Friesen <chris.friesen@...driver.com>,
	Austin Schuh <austin@...oton-tech.com>, pavel@...linux.ru,
	"J. Bruce Fields" <bfields@...ldses.org>,
	linux-ext4@...r.kernel.org, adilger.kernel@...ger.ca,
	rt-users <linux-rt-users@...r.kernel.org>
Subject: Re: RT/ext4/jbd2 circular dependency

On Wed, 29 Oct 2014, Theodore Ts'o wrote:
> On Wed, Oct 29, 2014 at 08:26:36PM +0100, Thomas Gleixner wrote:
> > > For what it's worth, I'm currently testing a backport of commit b34090e from
> > > mainline (which in turn required backporting commits e5a120a and f5113ef).  It
> > > switches from using the BJ_Shadow list to using the BH_Shadow flag on the
> > > buffer head.  More interestingly, waiters now get woken up from
> > > journal_end_buffer_io_sync() instead of from
> > > jbd2_journal_commit_transaction().
> > > 
> > > So far this seems to be helping a lot.  It's lasted about 15x as long under
> > > stress as without the patches.
> > 
> > I fear that this is just papering over the problem, but you have to
> > talk to the jbd2 folks about that.
> 
> No, it's a clean fix for the problem.  The main issue is that what the
> jbd2 commit was doing was starting inode writeback for those blocks
> needed to guarantee data=ordered mode (so this is what caused various
> pages to have writeback page set) as well as starting metadata writes
> to the commit (which is what caused the shadow bit to be set on the
> metadata buffers). 
> 
> Now that we clear the shadow flag when the metadata writes is
> complete, the writeback will eventually be allowed to complete and
> this prevents the deadlock.

That's a way better explanation than what I saw in the commit logs and
it actually maps to the observed traces and stackdumps.

Thanks for the clarification! I'm just getting nervous when 'picked
some backports' magically 'fixes' an issue without a proper
explanation.

Thanks,

       tglx
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