[PATCH 4/4] dma-buf: nuke reservation_object seq number

[Christian König]

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 at amd.com>
---
 drivers/dma-buf/reservation.c | 24 +++++-------------------
 include/linux/reservation.h   |  9 ++-------
 2 files changed, 7 insertions(+), 26 deletions(-)

diff --git a/drivers/dma-buf/reservation.c b/drivers/dma-buf/reservation.c
index 90bc6ef03598..f7f4a0858c2a 100644
--- a/drivers/dma-buf/reservation.c
+++ b/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);
-	/* write_seqcount_begin provides the necessary memory barrier */
-	RCU_INIT_POINTER(obj->fence_excl, fence);
+	rcu_assign_pointer(obj->fence_excl, fence);
+	/* pointer update must be visible before we modify the shared_count */
 	if (old)
-		old->shared_count = 0;
-	write_seqcount_end(&obj->seq);
+		smp_store_mb(old->shared_count, 0);
 	preempt_enable();
 
 	/* inplace update, no shared fences */
@@ -368,11 +357,8 @@ int reservation_object_copy_fences(struct reservation_object *dst,
 	old = reservation_object_get_excl(dst);
 
 	preempt_disable();
-	write_seqcount_begin(&dst->seq);
-	/* write_seqcount_begin provides the necessary memory barrier */
-	RCU_INIT_POINTER(dst->fence_excl, new);
-	RCU_INIT_POINTER(dst->fence, dst_list);
-	write_seqcount_end(&dst->seq);
+	rcu_assign_pointer(dst->fence_excl, new);
+	rcu_assign_pointer(dst->fence, dst_list);
 	preempt_enable();
 
 	reservation_object_list_free(src_list);
diff --git a/include/linux/reservation.h b/include/linux/reservation.h
index 044a5cd4af50..fd29baad0be3 100644
--- a/include/linux/reservation.h
+++ b/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;
@@ -156,14 +153,12 @@ 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);
 }
 
 /**
-- 
2.17.1