linux-ext4 - Re: [PATCH 1/4] ext4: Fix deadlock during page writeback

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Message-ID: <20160701165339.GB6075@thunk.org>
Date:	Fri, 1 Jul 2016 12:53:39 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Jan Kara <jack@...e.cz>
Cc:	linux-ext4@...r.kernel.org, Eryu Guan <eguan@...hat.com>,
	stable@...r.kernel.org
Subject: Re: [PATCH 1/4] ext4: Fix deadlock during page writeback

On Fri, Jul 01, 2016 at 11:09:50AM +0200, Jan Kara wrote:
> But it is not safe - the bio contains pages, those pages have PageWriteback
> set and if the inode is part of the running transaction,
> ext4_journal_stop() will wait for transaction commit which will wait for
> all outstanding writeback on the inode, which will deadlock on those pages
> which are part of our unsubmitted bio. So the ordering really has to be the
> way it is...

So to be clear. the issue is that PageWriteback won't get cleared
until we potentially do a uninit->init conversion, and this is what
requires taking a transaction handle leading to the other half of the
deadlock?

... and it's probably not safe to clear the PageWriteback early in the
bio completion callback function, isn't it.  Hmm....

						- Ted
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