dma-buf: Relax the write-seqlock for reallocating the shared fence list

As the set of shared fences is not being changed during reallocation of
the reservation list, we can skip updating the write_seqlock.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190712080314.21018-2-chris@chris-wilson.co.uk

drivers/dma-buf/reservation.c

@@ -157,15 +157,15 @@ int reservation_object_reserve_shared(struct reservation_object *obj,
 		(ksize(new) - offsetof(typeof(*new), shared)) /
 		sizeof(*new->shared);
 
-	preempt_disable();
-	write_seqcount_begin(&obj->seq);
 	/*
-	 * RCU_INIT_POINTER can be used here,
-	 * seqcount provides the necessary barriers
+	 * We are not changing the effective set of fences here so can
+	 * merely update the pointer to the new array; both existing
+	 * readers and new readers will see exactly the same set of
+	 * active (unsignaled) shared fences. Individual fences and the
+	 * old array are protected by RCU and so will not vanish under
+	 * the gaze of the rcu_read_lock() readers.
 	 */
-	RCU_INIT_POINTER(obj->fence, new);
-	write_seqcount_end(&obj->seq);
-	preempt_enable();
+	rcu_assign_pointer(obj->fence, new);
 
 	if (!old)
 		return 0;