[Intel-gfx] [PATCH 22/22] drm/i915: Export our request as a dma-buf fence on the reservation object

[Chris Wilson]

On Thu, Jul 28, 2016 at 02:28:10PM +0200, Daniel Vetter wrote:
> On Thu, Jul 28, 2016 at 01:17:39PM +0100, Chris Wilson wrote:
> > On Thu, Jul 28, 2016 at 01:59:45PM +0200, Daniel Vetter wrote:
> > > On Thu, Jul 28, 2016 at 11:40:29AM +0100, Chris Wilson wrote:
> > > > On Thu, Jul 28, 2016 at 12:32:42PM +0200, Daniel Vetter wrote:
> > > > > On Wed, Jul 27, 2016 at 12:15:00PM +0100, Chris Wilson wrote:
> > > > > > If the GEM objects being rendered with in this request have been
> > > > > > exported via dma-buf to a third party, hook ourselves into the dma-buf
> > > > > > reservation object so that the third party can serialise with our
> > > > > > rendering via the dma-buf fences.
> > > > > > 
> > > > > > Testcase: igt/prime_busy
> > > > > > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > > > > 
> > > > > Style nit: I prefer ww_mutex_lock(&resv->lock, NULL); over
> > > > > mutex_lock(&resv->lock.base). The former makes it clear it's a ww mutex,
> > > > > but we don't bother with the multi-lock dance. The latter needles around
> > > > > in implemenation details, which it really shouldn't. Please change.
> > > > 
> > > > Passing NULL as ww_acquite_ctx is illegal.
> > > 
> > > Hm, where exactly do you see that? kerneldoc for ww_mutex_lock clearly
> > > says that it can be NULL, and the static inline has a check for it and
> > > calls mutex_lock in the else path. Which means it /should/ boil down to
> > > the exact same code after gcc has pondered it enough.
> > 
> > But then explodes. Look at the lockdep. Clearly the kerneldoc is wrong.
> > Good job I was reading the code :-p
> 
> Hm, that'd be a bug in the ww_mutex_lock, and we make plenty use uf a NULL
> ctx in drm_modeset_lock.c. How exactly does this blow up? Can you attach
> the splat please?

&ctx->dep_map with a NULL pointer != 0 which blows up when it gets
deferenced inside lockdep, __lockdep_acquire():
        if (nest_lock && !__lock_is_held(nest_lock))
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre