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

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Date:   Thu, 11 Jun 2020 12:37:09 +0200
From:   Jan Kara <jack@...e.cz>
To:     Lukas Czerner <lczerner@...hat.com>
Cc:     linux-ext4@...r.kernel.org, Jan Kara <jack@...e.cz>
Subject: Re: jbd2: can b_transaction be NULL in refile_buffer ?

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.

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