drm/i915: Protect access to driver and timeline name

Protect the access to driver and timeline name which otherwise could be
freed as dma-fence exported is signalling fences.

This prepares the code for incoming dma-fence API changes which will start
asserting these accesses are done from a RCU locked section.

Now that the safe access is handled in the dma-fence API, the external
callers such as sync_file, and our internal code paths, we can drop the
similar protection from i915_fence_get_timeline_name().

This prepares the code for incoming dma-fence API changes which will start
asserting these accesses are done from a RCU locked section.

Signed-off-by: Tvrtko Ursulin <[e-mail]>
Reviewed-by: Andi Shyti <[e-mail]> # v1
Signed-off-by: Tvrtko Ursulin <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/gpu/drm/i915/gt/intel_gt_requests.c

···
 	llist_for_each_entry_safe(rq, rn, first, watchdog.link) {
 		if (!i915_request_completed(rq)) {
 			struct dma_fence *f = &rq->fence;
+			const char __rcu *timeline;
+			const char __rcu *driver;
 
+			rcu_read_lock();
+			driver = dma_fence_driver_name(f);
+			timeline = dma_fence_timeline_name(f);
 			pr_notice("Fence expiration time out i915-%s:%s:%llx!\n",
-				  dma_fence_driver_name(f),
-				  dma_fence_timeline_name(f),
+				  rcu_dereference(driver),
+				  rcu_dereference(timeline),
 				  f->seqno);
+			rcu_read_unlock();
 			i915_request_cancel(rq, -EINTR);
 		}
 		i915_request_put(rq);

drivers/gpu/drm/i915/i915_request.c

···
 		       const char *prefix,
 		       int indent)
 {
-	const char *name = dma_fence_timeline_name((struct dma_fence *)&rq->fence);
+	const char __rcu *timeline;
 	char buf[80] = "";
 	int x = 0;
 
···
 
 	x = print_sched_attr(&rq->sched.attr, buf, x, sizeof(buf));
 
+	rcu_read_lock();
+	timeline = dma_fence_timeline_name((struct dma_fence *)&rq->fence);
 	drm_printf(m, "%s%.*s%c %llx:%lld%s%s %s @ %dms: %s\n",
 		   prefix, indent, "                ",
 		   queue_status(rq),
···
 		   fence_status(rq),
 		   buf,
 		   jiffies_to_msecs(jiffies - rq->emitted_jiffies),
-		   name);
+		   rcu_dereference(timeline));
+	rcu_read_unlock();
 }
 
 static bool engine_match_ring(struct intel_engine_cs *engine, struct i915_request *rq)

drivers/gpu/drm/i915/i915_sw_fence.c

···
 	struct i915_sw_dma_fence_cb_timer *cb = timer_container_of(cb, t,
 								   timer);
 	struct i915_sw_fence *fence;
+	const char __rcu *timeline;
+	const char __rcu *driver;
 
 	fence = xchg(&cb->base.fence, NULL);
 	if (!fence)
 		return;
 
+	rcu_read_lock();
+	driver = dma_fence_driver_name(cb->dma);
+	timeline = dma_fence_timeline_name(cb->dma);
 	pr_notice("Asynchronous wait on fence %s:%s:%llx timed out (hint:%ps)\n",
-		  dma_fence_driver_name(cb->dma),
-		  dma_fence_timeline_name(cb->dma),
+		  rcu_dereference(driver),
+		  rcu_dereference(timeline),
 		  cb->dma->seqno,
 		  i915_sw_fence_debug_hint(fence));
+	rcu_read_unlock();
 
 	i915_sw_fence_set_error_once(fence, -ETIMEDOUT);
 	i915_sw_fence_complete(fence);