Re: [PATCH] rte_ring: fix racy dequeue/enqueue in ppc64

[Jerin Jacob <jerin.jacob@caviumnetworks.com>]

-----Original Message-----
> Date: Thu, 12 Jul 2018 11:44:14 +0900
> From: Takeshi Yoshimura <t.yoshimura8869@gmail.com>
> To: dev@dpdk.org
> Cc: Takeshi Yoshimura <t.yoshimura8869@gmail.com>, stable@dpdk.org, Takeshi
>  Yoshimura <tyos@jp.ibm.com>
> Subject: [dpdk-dev] [PATCH] rte_ring: fix racy dequeue/enqueue in ppc64
> X-Mailer: git-send-email 2.15.1
> 
> External Email
> 
> SPDK blobfs encountered a crash around rte_ring dequeues in ppc64.
> It uses a single consumer and multiple producers for a rte_ring.
> The problem was a load-load reorder in rte_ring_sc_dequeue_bulk().

Adding rte_smp_rmb() cause performance regression on non x86 platforms.
Having said that, load-load barrier can be expressed very  well with C11 memory
model. I guess ppc64 supports C11 memory model. If so,
Could you try CONFIG_RTE_RING_USE_C11_MEM_MODEL=y for ppc64 and check
original issue?

> 
> The reordered loads happened on r->prod.tail in
> __rte_ring_move_cons_head() (rte_ring_generic.h) and ring[idx] in
> DEQUEUE_PTRS() (rte_ring.h). They have a load-load control
> dependency, but the code does not satisfy it. Note that they are
> not reordered if __rte_ring_move_cons_head() with is_sc != 1 because
> cmpset invokes a read barrier.
> 
> The paired stores on these loads are in ENQUEUE_PTRS() and
> update_tail(). Simplified code around the reorder is the following.
> 
> Consumer             Producer
> load idx[ring]
>                      store idx[ring]
>                      store r->prod.tail
> load r->prod.tail
> 
> In this case, the consumer loads old idx[ring] and confirms the load
> is valid with the new r->prod.tail.
> 
> I added a read barrier in the case where __IS_SC is passed to
> __rte_ring_move_cons_head(). I also fixed __rte_ring_move_prod_head()
> to avoid similar problems with a single producer.
> 
> Cc: stable@dpdk.org
> 
> Signed-off-by: Takeshi Yoshimura <tyos@jp.ibm.com>
> ---
>  lib/librte_ring/rte_ring_generic.h | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/lib/librte_ring/rte_ring_generic.h b/lib/librte_ring/rte_ring_generic.h
> index ea7dbe5b9..477326180 100644
> --- a/lib/librte_ring/rte_ring_generic.h
> +++ b/lib/librte_ring/rte_ring_generic.h
> @@ -90,9 +90,10 @@ __rte_ring_move_prod_head(struct rte_ring *r, unsigned int is_sp,
>                         return 0;
> 
>                 *new_head = *old_head + n;
> -               if (is_sp)
> +               if (is_sp) {
> +                       rte_smp_rmb();
>                         r->prod.head = *new_head, success = 1;
> -               else
> +               } else
>                         success = rte_atomic32_cmpset(&r->prod.head,
>                                         *old_head, *new_head);
>         } while (unlikely(success == 0));
> @@ -158,9 +159,10 @@ __rte_ring_move_cons_head(struct rte_ring *r, unsigned int is_sc,
>                         return 0;
> 
>                 *new_head = *old_head + n;
> -               if (is_sc)
> +               if (is_sc) {
> +                       rte_smp_rmb();
>                         r->cons.head = *new_head, success = 1;
> -               else
> +               } else
>                         success = rte_atomic32_cmpset(&r->cons.head, *old_head,
>                                         *new_head);
>         } while (unlikely(success == 0));
> --
> 2.17.1
>