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@xxxxxxx>
> ---
> 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
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