drm/panthor: fix for dma-fence safe access rules

[ Upstream commit efe2489 ]

Commit 506aa8b ("dma-fence: Add safe access helpers and document
the rules") details the dma-fence safe access rules. The most common
culprit is that drm_sched_fence_get_timeline_name may race with
group_free_queue.

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Cc: stable@vger.kernel.org # v6.17+
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patch.msgid.link/20251204174545.399059-1-olvaffe@gmail.com
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: olvaffe

drivers/gpu/drm/panthor/panthor_sched.c

@@ -22,6 +22,7 @@
 #include <linux/module.h>
 #include <linux/platform_device.h>
 #include <linux/pm_runtime.h>
+#include <linux/rcupdate.h>
 
 #include "panthor_devfreq.h"
 #include "panthor_device.h"
@@ -926,6 +927,9 @@ static void group_release_work(struct work_struct *work)
 						   release_work);
 	u32 i;
 
+	/* dma-fences may still be accessing group->queues under rcu lock. */
+	synchronize_rcu();
+
 	for (i = 0; i < group->queue_count; i++)
 		group_free_queue(group, group->queues[i]);