Re: [lockdep] b09be676e0 BUG: unable to handle kernel NULL pointer dereference at 000001f2

[Byungchul Park <byungchul.park@lge.com>]

On Tue, Oct 10, 2017 at 09:56:26AM -0700, Linus Torvalds wrote:
> On Tue, Oct 10, 2017 at 9:22 AM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > I really would like to see the sites that do cross-thread lock/unlock
> > pairs themselves be annotated.
> >
> > So when you lock in one thread, and then unlock in another, I'd
> > actually prefer to see something like
> >
> >  - T1:
> >         lock_mutex_cross();
> >
> >  - T2:
> >         unlock_mutex_cross();
> >
> > to make it very explicit that *these* particular lock/unlock
> > operations are the fancy ones.
> 
> Actually, let's make it even *more* obvious, and even easier for
> lockdep (and for humans) to see what's going on.
> 
> So I think the best model would be something like this:
> 
>  - T1:
>         mutex_lock(&lock)
>         ...
>         mutex_transfer(&lock)
> 
>  - T2:
>         mutex_receive(&lock);
>         ...
>         mutex_unlock(&lock);

In addition, if we really need to use mutex locks in this way, I also
think it should work with the explicit primitives as you said.