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

Jan Kara
2015-08-13T10:02:08Z

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.

Once this is updated, feel free to add:

Reviewed-by: Jan Kara <jack@suse.com>

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