jbd2: Fix oops in jbd2_journal_remove_journal_head()

jbd2_journal_remove_journal_head() can oops when trying to access
journal_head returned by bh2jh(). This is caused for example by the
following race:

	TASK1					TASK2
  jbd2_journal_commit_transaction()
    ...
    processing t_forget list
      __jbd2_journal_refile_buffer(jh);
      if (!jh->b_transaction) {
        jbd_unlock_bh_state(bh);
					jbd2_journal_try_to_free_buffers()
					  jbd2_journal_grab_journal_head(bh)
					  jbd_lock_bh_state(bh)
					  __journal_try_to_free_buffer()
					  jbd2_journal_put_journal_head(jh)
        jbd2_journal_remove_journal_head(bh);

jbd2_journal_put_journal_head() in TASK2 sees that b_jcount == 0 and
buffer is not part of any transaction and thus frees journal_head
before TASK1 gets to doing so. Note that even buffer_head can be
released by try_to_free_buffers() after
jbd2_journal_put_journal_head() which adds even larger opportunity for
oops (but I didn't see this happen in reality).

Fix the problem by making transactions hold their own journal_head
reference (in b_jcount). That way we don't have to remove journal_head
explicitely via jbd2_journal_remove_journal_head() and instead just
remove journal_head when b_jcount drops to zero. The result of this is
that [__]jbd2_journal_refile_buffer(),
[__]jbd2_journal_unfile_buffer(), and
__jdb2_journal_remove_checkpoint() can free journal_head which needs
modification of a few callers. Also we have to be careful because once
journal_head is removed, buffer_head might be freed as well. So we
have to get our own buffer_head reference where it matters.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 0 insertions, 2 deletions

diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
index 4ecb7b16b278..d087c2e7b2aa 100644
--- a/include/linux/jbd2.h
+++ b/include/linux/jbd2.h
@@ -1024,7 +1024,6 @@ struct journal_s
 
 /* Filing buffers */
 extern void jbd2_journal_unfile_buffer(journal_t *, struct journal_head *);
-extern void __jbd2_journal_unfile_buffer(struct journal_head *);
 extern void __jbd2_journal_refile_buffer(struct journal_head *);
 extern void jbd2_journal_refile_buffer(journal_t *, struct journal_head *);
 extern void __jbd2_journal_file_buffer(struct journal_head *, transaction_t *, int);
@@ -1165,7 +1164,6 @@ extern void	   jbd2_journal_release_jbd_inode(journal_t *journal, struct jbd2_in
  */
 struct journal_head *jbd2_journal_add_journal_head(struct buffer_head *bh);
 struct journal_head *jbd2_journal_grab_journal_head(struct buffer_head *bh);
-void jbd2_journal_remove_journal_head(struct buffer_head *bh);
 void jbd2_journal_put_journal_head(struct journal_head *jh);
 
 /*