From mboxrd@z Thu Jan  1 00:00:00 1970
From: Justin He <Justin.He@arm.com>
Subject: Re: [PATCH v3 1/3] ring: read tail using atomic load
Date: Wed, 26 Sep 2018 10:09:44 +0000
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Cc: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>, Steve Capper
 <Steve.Capper@arm.com>, Ola Liljedahl <Ola.Liljedahl@arm.com>,
 "jerin.jacob@caviumnetworks.com" <jerin.jacob@caviumnetworks.com>, nd
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To: "Gavin Hu (Arm Technology China)" <Gavin.Hu@arm.com>, "dev@dpdk.org"
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> -----Original Message-----
> From: Gavin Hu (Arm Technology China)
> Sent: 2018=1B$BG/=1B(B9=1B$B7n=1B(B26=1B$BF|=1B(B 17:29
> To: Gavin Hu (Arm Technology China) <Gavin.Hu@arm.com>; dev@dpdk.org
> Cc: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>; Steve Capper
> <Steve.Capper@arm.com>; Ola Liljedahl <Ola.Liljedahl@arm.com>;
> jerin.jacob@caviumnetworks.com; nd <nd@arm.com>; stable@dpdk.org; Justin
> He <Justin.He@arm.com>
> Subject: RE: [PATCH v3 1/3] ring: read tail using atomic load
>
> +Justin He
>
> > -----Original Message-----
> > From: Gavin Hu <gavin.hu@arm.com>
> > Sent: Monday, September 17, 2018 4:17 PM
> > To: dev@dpdk.org
> > Cc: Gavin Hu (Arm Technology China) <Gavin.Hu@arm.com>; Honnappa
> > Nagarahalli <Honnappa.Nagarahalli@arm.com>; Steve Capper
> > <Steve.Capper@arm.com>; Ola Liljedahl <Ola.Liljedahl@arm.com>;
> > jerin.jacob@caviumnetworks.com; nd <nd@arm.com>; stable@dpdk.org
> > Subject: [PATCH v3 1/3] ring: read tail using atomic load
> >
> > 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 !=3D old_val))
> > +while (unlikely(old_val !=3D __atomic_load_n(&ht->tail,
> > +__ATOMIC_RELAXED)))

I still wonder why an atomic load is needed here?
Cheers,
Justin (Jia He)
> >  rte_pause();
> >
> >  __atomic_store_n(&ht->tail, new_val, __ATOMIC_RELEASE);
> > --
> > 2.7.4

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