[PATCH 8/8] dma-buf: nuke reservation_object seq number

[Chris Wilson]

Quoting Christian König (2019-08-06 16:01:34)
> 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.
> 
> Signed-off-by: Christian König <christian.koenig at amd.com>
> ---
>  drivers/dma-buf/reservation.c | 20 +++-----------------
>  include/linux/reservation.h   |  9 ++-------
>  2 files changed, 5 insertions(+), 24 deletions(-)
> 
> diff --git a/drivers/dma-buf/reservation.c b/drivers/dma-buf/reservation.c
> index 839d72af7ad8..43549a4d6658 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);

I think, now has to be rcu_assign_pointer.

 * Initialize an RCU-protected pointer in special cases where readers
 * do not need ordering constraints on the CPU or the compiler.  These
 * special cases are:
 *
 * 1.   This use of RCU_INIT_POINTER() is NULLing out the pointer *or*
 * 2.   The caller has taken whatever steps are required to prevent
 *      RCU readers from concurrently accessing this pointer *or*
 * 3.   The referenced data structure has already been exposed to
 *      readers either at compile time or via rcu_assign_pointer() *and*
 *
 *      a.      You have not made *any* reader-visible changes to
 *              this structure since then *or*
 *      b.      It is OK for readers accessing this structure from its
 *              new location to see the old state of the structure.  (For
 *              example, the changes were to statistical counters or to
 *              other state where exact synchronization is not required.)

We used to apply 2 from the seqcount, and 3 does not apply here.

> +       /* 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);

Hmm. Ok, I think.

>  {
> -       unsigned int seq;
> -
>         do {
> -               seq = read_seqcount_begin(&obj->seq);
>                 *excl = rcu_dereference(obj->fence_excl);
>                 *list = rcu_dereference(obj->fence);
> -       } while (read_seqcount_retry(&obj->seq, seq));
> +               smp_rmb(); /* See reservation_object_add_excl_fence */
> +       } while (rcu_access_pointer(obj->fence_excl) != *excl);
>  }

So, if we are add_excl_fence and cancelling a shared-list, before we
return we guarantee that the shared-list is set to zero if excl is set
as we read.

If we add to shared-list during the read, ... Hmm, actually we should
return num_list, i.e.

do {
	*list = rcu_dereference(obj->fence);
	num_list = *list ? (*list)->count : 0;
	smp_rmb();
} while (...)

return num_list.

as the relationship between the count and the fence entries is also
determined by the mb in add_shared_fence.

Oops, that was an oversight in the previous review.

>         preempt_enable();
>  
>         /* inplace update, no shared fences */
> @@ -370,11 +359,8 @@ 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_assign_pointer again I believe.
-Chris