Re: [PATCH v2 2/8] fix the broken lockdep logic in __sb_start_write()

Jan Kara
2015-08-13T13:29:49Z

On Thu 13-08-15 15:22:23, Oleg Nesterov wrote:
> On 08/13, Jan Kara wrote:
> >
> > On Tue 11-08-15 19:04:01, Oleg Nesterov wrote:
> > > 1. wait_event(frozen < level) without rwsem_acquire_read() is just
> > > wrong from lockdep perspective. If we are going to deadlock
> > > because the caller is buggy, lockdep detect this problem.
> > >
> > > 2. __sb_start_write() can race with thaw_super() + freeze_super(),
> > > and after "goto retry" the 2nd acquire_freeze_lock() is wrong.
> > >
> > > 3. The "tell lockdep we are doing trylock" hack doesn't look nice.
> > >
> > > I think this is correct, but this logic should be more explicit.
> > > Yes, the recursive read_lock() is fine if we hold the lock on a
> > > higher level. But we do not need to fool lockdep. If we can not
> > > deadlock in this case then try-lock must not fail and we can use
> > > use wait == F throughout this code.
> > >
> > > Note: as Dave Chinner explains, the "trylock" hack and the fat comment
> > > can be probably removed. But this needs a separate change and it will
> > > be trivial: just kill __sb_start_write() and rename do_sb_start_write()
> > > back to __sb_start_write().
> > >
> > > Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> >
> > Just a nit below...
> >
> > > + if (wait)
> > > + rwsem_acquire_read(&sb->s_writers.lock_map[level-1], 0, 0, ip);
> >
> > If we provided also __sb_writers_acquire() helper (in addition to _nowait)
> > variant, we could use these helpers in __sb_start_write() /
> > __sb_end_write() as well which would look better to me when we already have
> > them.
>
> Why? This code goes away after 8/8.

Right, OK. Objection retracted :).

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