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

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Message-ID: <20200611083417.4akdykeubd7kfuuh@work>
Date:   Thu, 11 Jun 2020 10:34:17 +0200
From:   Lukas Czerner <lczerner@...hat.com>
To:     linux-ext4@...r.kernel.org
Cc:     Jan Kara <jack@...e.cz>
Subject: jbd2: can b_transaction be NULL in refile_buffer ?

Hi,

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().

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 ?

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 ?

Thanks!
-Lukas

-- 

diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
index e91aad3637a2..55e5cb6b4bb5 100644
--- a/fs/jbd2/transaction.c
+++ b/fs/jbd2/transaction.c
@@ -2498,7 +2498,7 @@ void __jbd2_journal_file_buffer(struct journal_head *jh,
                __jbd2_journal_temp_unlink_buffer(jh);
        else
                jbd2_journal_grab_journal_head(bh);
-       jh->b_transaction = transaction;
+       WRITE_ONCE(jh->b_transaction, transaction);

        switch (jlist) {
        case BJ_None:
@@ -2577,15 +2577,14 @@ bool __jbd2_journal_refile_buffer(struct journal_head *jh)
         * our jh reference and thus __jbd2_journal_file_buffer() must not
         * take a new one.
         */
-       WRITE_ONCE(jh->b_transaction, jh->b_next_transaction);
-       WRITE_ONCE(jh->b_next_transaction, NULL);
        if (buffer_freed(bh))
                jlist = BJ_Forget;
        else if (jh->b_modified)
                jlist = BJ_Metadata;
        else
                jlist = BJ_Reserved;
-       __jbd2_journal_file_buffer(jh, jh->b_transaction, jlist);
+       __jbd2_journal_file_buffer(jh, jh->b_next_transaction, jlist);
+       WRITE_ONCE(jh->b_next_transaction, NULL);
        J_ASSERT_JH(jh, jh->b_transaction->t_state == T_RUNNING);

        if (was_dirty)
(END)