[Intel-gfx] [PATCH] drm: Assert correct locking for drm_send_vblank_event

[Daniel Vetter]

On Fri, Sep 12, 2014 at 01:03:51PM -0400, Peter Hurley wrote:
> On 09/12/2014 12:04 PM, Chris Wilson wrote:
> > On Fri, Sep 12, 2014 at 05:34:56PM +0200, Daniel Vetter wrote:
> >> On Fri, Sep 12, 2014 at 04:23:29PM +0100, Chris Wilson wrote:
> >>> On Fri, Sep 12, 2014 at 03:40:56PM +0200, Daniel Vetter wrote:
> >>>> The comment says that the caller must hold the dev->event_lock
> >>>> spinlock, so let's enforce this.
> >>>>
> >>>> A quick audit over all driver shows that except for the one place in
> >>>> i915 which motivated this all callers fullfill this requirement
> >>>> already.
> >>>
> >>> Replace the rogue WARN_ON_SMP(!spin_is_locked(&dev->event_lock)) in
> >>> send_vblank_event() as well then.
> >>
> >> Meh, I've missed that one, that's actually better I think. I'll drop my
> >> patch here.
> > 
> > I thought assert_spin_lock was the preferred form?
> 
> Actually, lockdep_assert_held() is the preferred form.
> 
> See https://lkml.org/lkml/2014/9/3/171

Which unfortunately doesn't warn for all the normal users which are not
insane enough to enable lockdep and so is totally useless to validate a
driver that runs on metric piles of different chips (with a resulting
combinatorial explosion of code-paths because hw designers are creative).
And we rely a lot on random drive-by testers to report such issues.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch