[Intel-gfx] ✗ Fi.CI.IGT: failure for drm/i915/userptr: Wrap mmu_notifier inside its own rw_semaphore

[Chris Wilson]

Quoting Patchwork (2018-03-26 17:53:44)
> Test gem_userptr_blits:
>         Subgroup coherency-unsync:
>                 pass       -> INCOMPLETE (shard-hsw)

Forgot that obj->userptr.mn may not exist.

>         Subgroup dmabuf-sync:
>                 pass       -> DMESG-WARN (shard-hsw)

But this is the tricky lockdep one, warning of the recursion from gup
into mmu_invalidate_range, i.e.

down_read(&i915_mmu_notifier->sem);
down_read(&mm_struct->mmap_sem);
	gup();
		down_write(&i915_mmut_notifier->sem);

That seems a genuine deadlock... So I wonder how we managed to get a
lockdep splat and not a dead machine. Maybe gup never triggers the
recursion for our set of flags? Hmm.
-Chris