[Intel-gfx] [PATCH 2/2] drm/i915: Initialise userptr mmu_notifier serial to 1

[Chris Wilson]

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 at chris-wilson.co.uk>
> >Cc: Tvrtko Ursulin <tvrtko.ursulin at 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 at 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.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre