FAILED: patch "[PATCH] drm/i915: fix incorrect RCU teardown order" failed to apply to 7.3-stable tree

FAILED: patch "[PATCH] drm/i915: fix incorrect RCU teardown order" failed to apply to 7.3-stable tree

[Sasha Levin]

The patch below does not apply to the 7.3-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-7.3.y
git checkout FETCH_HEAD
git cherry-pick -x 5113479556025093bf8133bb2dcaa33be2d50921
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107050318-failed-7.3-511347955602@kernel.org' --subject-prefix 'PATCH 7.3.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 5113479556025093bf8133bb2dcaa33be2d50921 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Christian=20K=C3=B6nig?= <ckoenig.leichtzumerken@gmail.com>
Date: Thu, 3 Sep 2026 13:36:21 +0200
Subject: [PATCH] drm/i915: fix incorrect RCU teardown order
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

i915_gem_busy_ioctl uses dma_resv_for_each_fence_unlocked() 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 i915_gem_busy_ioctl is still running. This won't free the GEM
objects memory, but still drops all the dma_fence references.

Now when dma_resv_for_each_fence_unlocked() sees a destroyed dma_fence 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.

Signed-off-by: Christian König <christian.koenig@amd.com>
Fixes: 912ff2ebd695 ("drm/i915: use the new iterator in i915_gem_busy_ioctl v2")
CC: stable@vger.kernel.org
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>
Link: https://lore.kernel.org/r/20260903113621.54660-1-christian.koenig@amd.com
---
 drivers/gpu/drm/i915/gem/i915_gem_object.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.c b/drivers/gpu/drm/i915/gem/i915_gem_object.c
index 5172d39826548..9e01f8b2079ab 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_object.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_object.c
@@ -89,6 +89,7 @@ struct drm_i915_gem_object *i915_gem_object_alloc(void)
 
 void i915_gem_object_free(struct drm_i915_gem_object *obj)
 {
+	dma_resv_fini(&obj->base._resv);
 	return kmem_cache_free(slab_objects, obj);
 }
 
@@ -144,7 +145,6 @@ void __i915_gem_object_fini(struct drm_i915_gem_object *obj)
 {
 	mutex_destroy(&obj->mm.get_page.lock);
 	mutex_destroy(&obj->mm.get_dma_page.lock);
-	dma_resv_fini(&obj->base._resv);
 }
 
 /**
-- 
2.53.0