> -----Original Message-----
> From: Eads, Gage
> Sent: Friday, March 29, 2019 2:25 PM
> To: Honnappa Nagarahalli <honnappa.nagaraha...@arm.com>;
> dev@dpdk.org
> Cc: olivier.m...@6wind.com; arybche...@solarflare.com; Richardson, Bruce
> <bruce.richard...@intel.com>; Ananyev, Konstantin
> <konstantin.anan...@intel.com>; Gavin Hu (Arm Technology China)
> <gavin...@arm.com>; nd <n...@arm.com>; tho...@monjalon.net; nd
> <n...@arm.com>; Thomas Monjalon <tho...@monjalon.net>
> Subject: RE: [PATCH v3 6/8] stack: add C11 atomic implementation
>
> [snip]
>
> > > +static __rte_always_inline void
> > > +__rte_stack_lf_push(struct rte_stack_lf_list *list,
> > > + struct rte_stack_lf_elem *first,
> > > + struct rte_stack_lf_elem *last,
> > > + unsigned int num)
> > > +{
> > > +#ifndef RTE_ARCH_X86_64
> > > + RTE_SET_USED(first);
> > > + RTE_SET_USED(last);
> > > + RTE_SET_USED(list);
> > > + RTE_SET_USED(num);
> > > +#else
> > > + struct rte_stack_lf_head old_head;
> > > + int success;
> > > +
> > > + old_head = list->head;
> > This can be a torn read (same as you have mentioned in
> > __rte_stack_lf_pop). I suggest we use acquire thread fence here as
> > well (please see the comments in __rte_stack_lf_pop).
>
> Agreed. I'll add the acquire fence.
>
On second thought, an acquire fence isn't necessary. The acquire fence in
__rte_stack_lf_pop() ensures the list->head is ordered before the list element
reads. That isn't necessary here; we need to ensure that the last->next write
occurs (and is observed) before the list->head write, which the CAS's RELEASE
success memorder accomplishes.
If a torn read occurs, the CAS will fail and will atomically re-load &old_head.
