On Jun 15, 2015, at 7:06 AM, Theodore Ts'o <tytso@xxxxxxx> 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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