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

[Chris Wilson]

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 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);
>  }

Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>

I think this is correct. Now see if we can convince Daniel!
-Chris