dma-buf: Provide wrappers for reservation's lock

Joonas complained that writing ww_mutex_lock(&resv->lock, ctx) was too
intrusive compared to reservation_object_lock(resv, ctx);

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161115154642.31850-1-chris@chris-wilson.co.uk

include/linux/reservation.h

@@ -144,6 +144,40 @@ reservation_object_get_list(struct reservation_object *obj)
 					 reservation_object_held(obj));
 }
 
+/**
+ * reservation_object_lock - lock the reservation object
+ * @obj: the reservation object
+ * @ctx: the locking context
+ *
+ * Locks the reservation object for exclusive access and modification. Note,
+ * that the lock is only against other writers, readers will run concurrently
+ * with a writer under RCU. The seqlock is used to notify readers if they
+ * overlap with a writer.
+ *
+ * As the reservation object may be locked by multiple parties in an
+ * undefined order, a #ww_acquire_ctx is passed to unwind if a cycle
+ * is detected. See ww_mutex_lock() and ww_acquire_init(). A reservation
+ * object may be locked by itself by passing NULL as @ctx.
+ */
+static inline int
+reservation_object_lock(struct reservation_object *obj,
+			struct ww_acquire_ctx *ctx)
+{
+	return ww_mutex_lock(&obj->lock, ctx);
+}
+
+/**
+ * reservation_object_unlock - unlock the reservation object
+ * @obj: the reservation object
+ *
+ * Unlocks the reservation object following exclusive access.
+ */
+static inline void
+reservation_object_unlock(struct reservation_object *obj)
+{
+	ww_mutex_unlock(&obj->lock);
+}
+
 /**
  * reservation_object_get_excl - get the reservation object's
  * exclusive fence, with update-side lock held