linux-ext4 - Re: jbd2: can b_transaction be NULL in refile

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Date:   Thu, 11 Jun 2020 12:50:24 +0200
From:   Lukas Czerner <lczerner@...hat.com>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: jbd2: can b_transaction be NULL in refile_buffer ?

On Thu, Jun 11, 2020 at 12:37:09PM +0200, Jan Kara wrote:
> Hi!
> 
> On Thu 11-06-20 10:34:17, Lukas Czerner wrote:
> > I am tracking a rare and very hard to reproduce bug that ends up hittng
> > 
> > J_ASSERT_JH(jh, jh->b_transaction == NULL)
> > 
> > in __journal_remove_journal_head(). In fact we can get there with
> > b_next_transaction set and b_jlist == BJ_Forget so it's clear that we
> > should not have dropped the last JH reference at that point.
> > 
> > Most of the time that I've seen we get there from
> > __jbd2_journal_remove_checkpoint() called from
> > jbd2_journal_commit_transaction().
> > 
> > The locking in and around grabbing and putting the journal head
> > reference (b_jcount) looks solid as well as the use of j_list_lock. But
> > I have noticed a problem in logic of __jbd2_journal_refile_buffer().
> 
> Yeah, the trouble with refcounting bugs is that if *any* of the users
> releases a reference it should not, we will (much later) hit the problem you
> describe.
> 
> > The idea is that b_next_transaction will inherit the reference from
> > b_transaction so that we do not need to grab a new reference of
> > journal_head. However this will only be true if b_transaction is set.
> >
> > But if it is indeed NULL, then we will do
> > 
> > WRITE_ONCE(jh->b_transaction, jh->b_next_transaction);
> > 
> > and __jbd2_journal_file_buffer() will not grab the jh reference. AFAICT
> > the b_next_transaction is not holding it's own jh reference. This will
> > result in b_transaction _not_ holding it's own jh reference and we will
> > be able to drop the last jh reference at unexpected places - hence we can
> > hit the asserts in __journal_remove_journal_head().
> >
> > However I am not really sure if it is indeed possible to get into
> > __jbd2_journal_refile_buffer() with b_transaction == NULL and
> > b_next_transaction set. Jan do you have any idea if that's possible and
> > what would be the circumstances to lead us there ?
> 
> __jbd2_journal_refile_buffer() should always be called with b_transaction
> != NULL and as I've checked (all three) callers, that indeed seems to be
> the case. Feel free to add assert along those lines to
> __jbd2_journal_refile_buffer() to see whether it triggers...
> 
> > Regardless I still think this is a bug in the logic and we should either
> > make sure that b_transaction is _not_ NULL in
> > __jbd2_journal_refile_buffer(), or let __jbd2_journal_file_buffer() grab
> > the jh reference if b_transaction was indeen NULL. How about something
> > like the following untested patch ?
> 
> I'd rather got for the assert. It makes things simpler, also the "meaning"
> of __jbd2_journal_refile_buffer() is "jh is done in its current
> transaction, deal with it" and that doesn't have a great meaning if
> b_transaction is NULL.
> 
> And when you're adding asserts, then adding one in
> __jbd2_journal_unfile_buffer() checking b_transaction != NULL and
> b_next_transaction == NULL would be good as well because lot of callers
> assume this. I've checked the code and I didn't find any problematic one
> but that code is complex enough that I could have missed something.

Ok, thanks. I'll prepare a patch.

Regards,
-Lukas

> 
> 								Honza
> -- 
> Jan Kara <jack@...e.com>
> SUSE Labs, CR
>