[dpdk-dev] [PATCH v4 2/4] ring: read tail using atomic load

Gavin Hu (Arm Technology China) Gavin.Hu at arm.com
Tue Sep 25 11:26:34 CEST 2018


+ Justin He as Jerin requested. 

> -----Original Message-----
> From: Jerin Jacob <jerin.jacob at caviumnetworks.com>
> Sent: Thursday, September 20, 2018 2:41 PM
> To: Gavin Hu (Arm Technology China) <Gavin.Hu at arm.com>
> Cc: dev at dpdk.org; Honnappa Nagarahalli
> <Honnappa.Nagarahalli at arm.com>; Steve Capper
> <Steve.Capper at arm.com>; Ola Liljedahl <Ola.Liljedahl at arm.com>; nd
> <nd at arm.com>; stable at dpdk.org; jia.he at hxt-semitech.com
> Subject: Re: [PATCH v4 2/4] ring: read tail using atomic load
> 
> -----Original Message-----
> > Date: Mon, 17 Sep 2018 16:11:17 +0800
> > From: Gavin Hu <gavin.hu at arm.com>
> > To: dev at dpdk.org
> > CC: gavin.hu at arm.com, Honnappa.Nagarahalli at arm.com,
> > steve.capper at arm.com,  Ola.Liljedahl at arm.com,
> > jerin.jacob at caviumnetworks.com, nd at arm.com,  stable at dpdk.org
> > Subject: [PATCH v4 2/4] ring: read tail using atomic load
> > X-Mailer: git-send-email 2.7.4
> >
> >
> > 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 at dpdk.org
> 
> 
> + Jia He <jia.he at hxt-semitech.com>
> 
> >
> > Signed-off-by: Gavin Hu <gavin.hu at arm.com>
> > Reviewed-by: Honnappa Nagarahalli <Honnappa.Nagarahalli at arm.com>
> > Reviewed-by: Steve Capper <steve.capper at arm.com>
> > Reviewed-by: Ola Liljedahl <Ola.Liljedahl at 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();
> >
> >         __atomic_store_n(&ht->tail, new_val, __ATOMIC_RELEASE);
> > --
> > 2.7.4
> >


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