dma-buf: nuke reservation_object seq number

The only remaining use for this is to protect against setting a new exclusive
fence while we grab both exclusive and shared. That can also be archived by
looking if the exclusive fence has changed or not after completing the
operation.

v2: switch setting excl fence to rcu_assign_pointer

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/322380/

drivers/dma-buf/reservation.c

@@ -49,12 +49,6 @@
 DEFINE_WD_CLASS(reservation_ww_class);
 EXPORT_SYMBOL(reservation_ww_class);
 
-struct lock_class_key reservation_seqcount_class;
-EXPORT_SYMBOL(reservation_seqcount_class);
-
-const char reservation_seqcount_string[] = "reservation_seqcount";
-EXPORT_SYMBOL(reservation_seqcount_string);
-
 /**
  * reservation_object_list_alloc - allocate fence list
  * @shared_max: number of fences we need space for
@@ -103,9 +97,6 @@ static void reservation_object_list_free(struct reservation_object_list *list)
 void reservation_object_init(struct reservation_object *obj)
 {
 	ww_mutex_init(&obj->lock, &reservation_ww_class);
-
-	__seqcount_init(&obj->seq, reservation_seqcount_string,
-			&reservation_seqcount_class);
 	RCU_INIT_POINTER(obj->fence, NULL);
 	RCU_INIT_POINTER(obj->fence_excl, NULL);
 }
@@ -282,12 +273,10 @@ void reservation_object_add_excl_fence(struct reservation_object *obj,
 		dma_fence_get(fence);
 
 	preempt_disable();
-	write_seqcount_begin(&obj->seq);
+	rcu_assign_pointer(obj->fence_excl, fence);
-	/* write_seqcount_begin provides the necessary memory barrier */
+	/* pointer update must be visible before we modify the shared_count */
-	RCU_INIT_POINTER(obj->fence_excl, fence);
 	if (old)
-		old->shared_count = 0;
+		smp_store_mb(old->shared_count, 0);
-	write_seqcount_end(&obj->seq);
 	preempt_enable();
 
 	/* inplace update, no shared fences */
@@ -368,11 +357,8 @@ retry:
 	old = reservation_object_get_excl(dst);
 
 	preempt_disable();
-	write_seqcount_begin(&dst->seq);
+	rcu_assign_pointer(dst->fence_excl, new);
-	/* write_seqcount_begin provides the necessary memory barrier */
+	rcu_assign_pointer(dst->fence, dst_list);
-	RCU_INIT_POINTER(dst->fence_excl, new);
-	RCU_INIT_POINTER(dst->fence, dst_list);
-	write_seqcount_end(&dst->seq);
 	preempt_enable();
 
 	reservation_object_list_free(src_list);

include/linux/reservation.h

@@ -46,8 +46,6 @@
 #include <linux/rcupdate.h>
 
 extern struct ww_class reservation_ww_class;
-extern struct lock_class_key reservation_seqcount_class;
-extern const char reservation_seqcount_string[];
 
 /**
  * struct reservation_object_list - a list of shared fences
@@ -71,7 +69,6 @@ struct reservation_object_list {
  */
 struct reservation_object {
 	struct ww_mutex lock;
-	seqcount_t seq;
 
 	struct dma_fence __rcu *fence_excl;
 	struct reservation_object_list __rcu *fence;
@@ -130,14 +127,12 @@ reservation_object_fences(struct reservation_object *obj,
 			  struct reservation_object_list **list,
 			  u32 *shared_count)
 {
-	unsigned int seq;
-
 	do {
-		seq = read_seqcount_begin(&obj->seq);
 		*excl = rcu_dereference(obj->fence_excl);
 		*list = rcu_dereference(obj->fence);
 		*shared_count = *list ? (*list)->shared_count : 0;
-	} while (read_seqcount_retry(&obj->seq, seq));
+		smp_rmb(); /* See reservation_object_add_excl_fence */
+	} while (rcu_access_pointer(obj->fence_excl) != *excl);
 }
 
 /**