Quoting Christian König (2019-08-07 14:53:12)
> 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@xxxxxxx>
> ---
> 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);
> }
Reviewed-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
I think this is correct. Now see if we can convince Daniel!
-Chris
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