On Mon, May 12, 2014 at 06:11:18PM +0200, Daniel Vetter wrote: > On Mon, May 12, 2014 at 09:05:45AM +0000, Mateo Lozano, Oscar wrote: > > Hi Daniel, > > > > Sorry, this fell through the cracks: > > > > > Subject: Re: [PATCH] drm/i915: Gracefully handle obj not bound to > > > GGTT in is_pin_display > > > > > > On Wed, Apr 02, 2014 at 07:21:01PM +0100, oscar.mateo@xxxxxxxxx wrote: > > > > From: Oscar Mateo <oscar.mateo@xxxxxxxxx> > > > > > > > > Otherwise, we do a NULL pointer dereference. > > > > > > > > I've seen this happen while handling an error in > > > > i915_gem_object_pin_to_display_plane(): > > > > > > > > If i915_gem_object_set_cache_level() fails, we call is_pin_display() > > > > to handle the error. At this point, the object is still not pinned to > > > > GGTT and maybe not even bound, so we have to check before we > > > > dereference its GGTT vma. > > > > > > > > Issue: VIZ-3772 > > > > Signed-off-by: Oscar Mateo <oscar.mateo@xxxxxxxxx> > > > > > > Have you looked into provoking this with an igt testcase? On a hunch a busy > > > load (to extend the race window) plus the usual interruptor trick to jump out of > > > wait_seqno calls should be able to make this go kaboom on command. But I > > > haven't analyzed the bug in detail. > > > > AFAICT, the only sequence where this likely to happen (because we are handling a recently created object) is: > > > > intelfb_alloc -> intel_pin_and_fence_fb_obj -> i915_gem_object_pin_to_display_plane -> i915_gem_object_set_cache_level -> is_pin_display > > Pageflipping to a freshly allocated BO without ever touching it beforehand > should be able to achive the same. If this is really all that's needed. > > But looking at the code a better way should be: > 1. Create new bo, wrap it in a kms fb. > 2. Slap busy load onto that bo, e.g. reapeatedly fill it with the blitter. > 3. Enable evil interruptor (igt_fork_signal_helper). > 4. Submit pageflip > > -> Boom since the set_cache_level will block, get interrupted and exit > early with -EINTR. > > Given sufficient overkill in 2. this should be 100% reliable to reproduce. Aside: Those kinds of tricks are a big reason why I think igts aren't just useful as testcases, but also to really understand how a bug comes about. At least ime finally figuring out the last ingredient to make an igt fully reliably often resulted in a suddenly much clearer understanding of the bug at hand. I call this "review by asking for an igt" ;-) -Daniel -- Daniel Vetter Software Engineer, Intel Corporation +41 (0) 79 365 57 48 - http://blog.ffwll.ch _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/intel-gfx
