CVE-2026-98521: drm/i915: fix incorrect RCU teardown order

In the Linux kernel, the following vulnerability has been resolved:

drm/i915: fix incorrect RCU teardown order

i915gembusyioctl uses dmaresvforeachfenceunlocked() to iterate over the fences in an GEM object without holding a reference but only the RCU read side lock.

What can happen here is that the GEM object is destroyed concurrently while i915gembusyioctl is still running. This won't free the GEM objects memory, but still drops all the dmafence references.

Now when dmaresvforeachfenceunlocked() sees a destroyed dmafence it assumes that a new fence list was installed and re-starts the loop.

But in the case of a destroyed GEM object a new fence list is never installed, only the old one freed and therefore the iteration never finishes resulting in an endless loop.

The solution is to drop the fence references only after the RCU grace period.

The fixes tag is not necessary the patch introducing the problem, but the one making it so worse that we need to address it.

This problem was pointed out by Sashiko-bot.

(cherry picked from commit 5113479556025093bf8133bb2dcaa33be2d50921)