linux-ext4 - Re: [PATCH] ext4: fix race between truncate and __ext4_journalled

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Date:	Mon, 15 Jun 2015 11:59:59 -0600
From:	Andreas Dilger <adilger@...ger.ca>
To:	Theodore Ts'o <tytso@....edu>
Cc:	Jan Kara <jack@...e.cz>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	enwlinux@...il.com, stable@...r.kernel.org
Subject: Re: [PATCH] ext4: fix race between truncate and __ext4_journalled_writepage()

On Jun 15, 2015, at 7:06 AM, Theodore Ts'o <tytso@....edu> wrote:
> 
> On Mon, Jun 15, 2015 at 02:33:52PM +0200, Jan Kara wrote:
>>  Yeah, that's nasty. Thanks for debugging this! However I think your fix
>> reintroduces the original deadlock issues. do_journal_get_write_access()
>> can end up blocking waiting for jbd2 thread to finish a commit while jbd2
>> thread may be blocked waiting for the page to be unlocked.
>> 
>> After some thought I don't think the deadlock is real since
>> do_journal_get_write_access() will currently only block if a buffer is
>> under writeout to the journal and at that point we don't wait for page
>> locks anymore. Also ext4_write_begin() does the same in data=journal mode
>> and we haven't observed deadlocks so far. But still things look really
>> fragile here.
> 
> The reason why there are no deadlocks is the writeback in the commit
> thread happens when the inode gets written back --- but that only
> happens for data=ordered inodes, not data=journalled mode.  I was a
> little worried about what might happen when after the 'j' chattr
> attribute gets set on an inode, and the inode was still on the ordered
> flush list.
> 
> Hmm... I think we could also maybe fix this by having
> ext4_change_inode_journal_flag() force a journal commit before setting
> the JOURNAL_DATA flag.  If we did that, we could just avoid dropping
> the page_lock in __ext4_journalled_writepage() altogether.

That is already being done:

int ext4_change_inode_journal_flag(struct inode *inode, int val)
{
        :
        :

        jbd2_journal_lock_updates(journal);

        :
        :


/**
 * void jbd2_journal_lock_updates () - establish a transaction barrier.
 * @journal:  Journal to establish a barrier on.
 *
 * This locks out any further updates from being started, and blocks
 * until all existing updates have completed, returning only once the
 * journal is in a quiescent state with no updates running.
 *
 * The journal lock should not be held on entry.
 */


Cheers, Andreas





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