Date:	Tue, 21 Aug 2007 18:48:15 +0200
From:	Jan Kara <jack@...e.cz>
To:	Josef Bacik <jbacik@...hat.com>
Cc:	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fix panic in jbd by adding locks

On Tue 21-08-07 11:43:12, Josef Bacik wrote:
> On Mon, Aug 20, 2007 at 05:20:21PM +0200, Jan Kara wrote:
> >   OK, thanks. So record probably points to an already freed memory which has
> > been overwritten by garbage...
> > 
> > > >   Thanks for details. I'm still not convinced. What they essentially
> > > > write is that slab cache revoke_record_cache is not guarded by any spin
> > > > lock. It's not and that should be fine as slab caches are SMP safe by
> > > > themselves.
> > > 
> > > No its the list_del thats not gaurded, so the hash list gets screwed up outside
> > > of a lock.  If there are other problems that need to be addressed then ok, but I
> > > still think that we should be protecting all of the list traversal/changing
> > > should be protected by the lock.  Thank you,
> >   But the traversal in journal_write_revoke_records() *is* in fact guarded
> > by the commit logic in journal_commit_transaction() handling code - it
> > doesn't allow anybody to mess with revoke lists when a transaction is
> > committing. So there's no need to guard the hash list again in
> > journal_write_revoke_records() by the spinlock. And if the logic does not
> > work and lets somebody modify revoke lists during commit, we have more
> > serious problems than hash list corruption. That's why I'm trying to find
> > out where's the real culprit of the Oops. But so far I cannot find out how
> > the corruption can happen...
> 
> I should note here I'm not trying to be argumentative, I just want to
> understand.  Ok so journal_commit_transaction() will make sure all the
  I also just want to understand how the oops can happen :).

> handle_t's are removed and such before processing the revoke lists, but right
> before we process the revoke lists we set the journals running transaction to
> NULL, which means we can continue on our merry way.  AFAICS the revoke lists are
> per journal, not per transaction, so once we give up the j_state_lock after
> having made sure the handle_t's had done their thing, we set the
> running_transaction to NULL letting people continue to do their thing, and since
> the revoke table is on a per journal basis, its completely valid for a new
> transaction to be started, a handle to be added to it, journal_cancel_revoke()
> to be run against that handle while still in journal_commit_transaction().  It
> wouldn't necessarily be a handle_t from the transaction we are in the middle of
> committing, it would be from a new transaction, and since the revoke list is per
> journal, both the transaction thats currently being committed and the new
> transaction would have access to the same revoke list, hence the race.  Is this
> correct?  If not let me know because I want to understand this code better.
  The trick is, there are in fact two revoke tables. So the commit code
does the following:
  1) It waits until all handles of the running transaction are released
(this is the while loop waiting checking t_updates).
  2) It does some cleanup of unused buffers.
  3) Switches revoke tables - i.e. journal->j_revoke now points to a
freshly initialized table, the table of the committing transaction is kept
hidden.
  4) Transaction state is changed to FLUSH, journal->j_running_transaction
is set to NULL, etc.
  5) Data writeout is performed.
  6) Saved revoke hash table is written.

  After 3), journal_revoke() and journal_revoke_cancel() access the new
hash table and thus have no influence on journal_write_revoke_records(). It
could possibly be that the pointer to the old hash table would be stored in
some local variable - but all the places where we store a pointer to the
hash table seem to be contained inside journal_start(), journal_stop()
pairs (all functions working with hash tables take a transaction handle
as an argument) and because all handles were released at some point in time,
we know that this should not happen either.
  I hope it is clearer now. If you still have any questions, please ask.

									Honza
-- 
Jan Kara <jack@...e.cz>
SuSE CR Labs
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