Re: [dpdk-dev] [PATCH v3 1/3] ring: read tail using atomic load

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

-----Original Message-----
> Date: Mon, 17 Sep 2018 16:17:22 +0800
> From: Gavin Hu <gavin.hu@arm.com>
> To: dev@dpdk.org
> CC: gavin.hu@arm.com, Honnappa.Nagarahalli@arm.com, steve.capper@arm.com,
>  Ola.Liljedahl@arm.com, jerin.jacob@caviumnetworks.com, nd@arm.com,
>  stable@dpdk.org
> Subject: [PATCH v3 1/3] ring: read tail using atomic load
> X-Mailer: git-send-email 2.7.4
> 
> External Email
> 
> In update_tail, read ht->tail using __atomic_load.Although the
> compiler currently seems to be doing the right thing even without
> _atomic_load, we don't want to give the compiler freedom to optimise
> what should be an atomic load, it should not be arbitarily moved
> around.
> 
> Fixes: 39368ebfc6 ("ring: introduce C11 memory model barrier option")
> Cc: stable@dpdk.org
> 
> Signed-off-by: Gavin Hu <gavin.hu@arm.com>
> Reviewed-by: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>
> Reviewed-by: Steve Capper <steve.capper@arm.com>
> Reviewed-by: Ola Liljedahl <Ola.Liljedahl@arm.com>
> ---
>  lib/librte_ring/rte_ring_c11_mem.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/librte_ring/rte_ring_c11_mem.h b/lib/librte_ring/rte_ring_c11_mem.h
> index 94df3c4..234fea0 100644
> --- a/lib/librte_ring/rte_ring_c11_mem.h
> +++ b/lib/librte_ring/rte_ring_c11_mem.h
> @@ -21,7 +21,8 @@ update_tail(struct rte_ring_headtail *ht, uint32_t old_val, uint32_t new_val,
>          * we need to wait for them to complete
>          */
>         if (!single)
> -               while (unlikely(ht->tail != old_val))
> +               while (unlikely(old_val != __atomic_load_n(&ht->tail,
> +                                               __ATOMIC_RELAXED)))
>                         rte_pause();

Since it is a while loop with rte_pause(), IMO, There is no scope of false compiler optimization.
IMO, this change may not required though I don't see any performance
difference with two core ring_perf_autotest test. May be more core
case it may have effect. IMO, If it not absolutely required, we can avoid
this change.

> 
>         __atomic_store_n(&ht->tail, new_val, __ATOMIC_RELEASE);
> --
> 2.7.4
>