Re: [PATCH 2/2] drm/i915: Initialise userptr mmu_notifier serial to 1

[Daniel Vetter <daniel@ffwll.ch>]

On Fri, Jul 11, 2014 at 12:06:02PM +0100, Chris Wilson wrote:
> On Fri, Jul 11, 2014 at 12:00:26PM +0100, Tvrtko Ursulin wrote:
> > 
> > On 07/11/2014 11:28 AM, Chris Wilson wrote:
> > >During the range invalidate, we walk the list of buffers associated with
> > >the mmu_notifer and find the ones that overlap the range. An
> > >optimisation is made to speed up the iteration by assuming the previous
> > >iter is still valid whilst the tree is unmodified. This exposes a bug
> > >when a range invalidate is triggered after we have just created the
> > >mmu_notifier, but before attaching any buffers. In that case, we presume
> > >we have an unmodified list and start walking from the last iter which is
> > >NULL. Oops.
> > >
> > >The easiest fix is then to initialise the serial of the tree to 1.
> > >
> > >Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > >Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
> > >---
> > >  drivers/gpu/drm/i915/i915_gem_userptr.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > >diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
> > >index 7c38f50014db..8e9e91029aed 100644
> > >--- a/drivers/gpu/drm/i915/i915_gem_userptr.c
> > >+++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
> > >@@ -197,7 +197,7 @@ i915_mmu_notifier_get(struct drm_device *dev, struct mm_struct *mm)
> > >  	mmu->mm = mm;
> > >  	mmu->objects = RB_ROOT;
> > >  	mmu->count = 0;
> > >-	mmu->serial = 0;
> > >+	mmu->serial = 1;
> > >  	INIT_LIST_HEAD(&mmu->linear);
> > >  	mmu->is_linear = false;
> > >
> > 
> > Looks good to me and I think safe to merge in any case, so:
> > 
> > Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> > 
> > But it will be interesting to know what code managed to trigger this
> > race, because as we discussed on IRC it would indicate some pretty
> > wild userspace behaviour. Or lack of imagination on our part?
> 
> A threaded client. One thread using userptr, the other doing munmap or
> free. Given enough embarrassment, it will happen every time.

Can I have an igt please to confirm this and ensure it stays fixed?
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch