RE: rte_ring move head question for machines with relaxed MO (arm/ppc)

[Wathsala Wathawana Vithanage <wathsala.vithanage@arm.com>]

> >
> > > > 1. rte_ring_generic_pvt.h:
> > > > =====================
> > > >
> > > > pseudo-c-code                                      //        related armv8 instructions
> > > > --------------------                                                 --------------------------------------
> > > >  head.load()                                          //        ldr [head]
> > > >  rte_smp_rmb()                                    //        dmb ishld
> > > >  opposite_tail.load()                            //        ldr [opposite_tail]
> > > >  ...
> > > >  rte_atomic32_cmpset(head, ...)      //        ldrex[head];... stlex[head]
> > > >
> > > >
> > > > 2. rte_ring_c11_pvt.h
> > > > =====================
> > > >
> > > > pseudo-c-code                                       //        related armv8 instructions
> > > > --------------------                                                 --------------------------------------
> > > > head.atomic_load(relaxed)                 //        ldr[head]
> > > > atomic_thread_fence(acquire)           //        dmb ish
> > > > opposite_tail.atomic_load(acquire)   //        lda[opposite_tail]
> > > > ...
> > > > head.atomic_cas(..., relaxed)              //        ldrex[haed]; ... strex[head]
> > > >
> > > >
> > > > 3.   rte_ring_hts_elem_pvt.h
> > > > ==========================
> > > >
> > > > pseudo-c-code                                       //        related armv8 instructions
> > > > --------------------                                                 --------------------------------------
> > > > head.atomic_load(acquire)                //        lda [head]
> > > > opposite_tail.load()                             //        ldr [opposite_tail]
> > > > ...
> > > > head.atomic_cas(..., acquire)            //         ldaex[head]; ... strex[head]
> > > >
> > > > The questions that arose from these observations:
> > > > a) are all 3 approaches equivalent in terms of functionality?
> > > Different, lda (Load with acquire semantics) and ldr (load) are different.
> >
> > I understand that, my question was:
> > lda {head]; ldr[tail]
> > vs
> > ldr [head]; dmb ishld; ldr [tail];
> >
> > Is there any difference in terms of functionality (memory ops
> ordering/observability)?
> 
> To be more precise:
> 
> lda {head]; ldr[tail]
> vs
> ldr [head]; dmb ishld; ldr [tail];
> vs
> ldr [head]; dmb ishld; lda [tail];
> 
> what would be the difference between these 3 cases?

Case A: lda {head]; ldr[tail]
load of the head will be observed by the memory subsystem
before the load of the tail.

Case B: ldr [head]; dmb ishld; ldr [tail];
load of the head will be observed by the memory subsystem
Before the load of the tail.

Case C: ldr [head]; dmb ishld; lda [tail];
load of the head will be observed by the memory subsystem
before the load of the tail. In addition, any load or store program
order after lda[tail] will not be observed by the memory subsystem
before the load of the tail.

Essentially both cases A and B are the same.
They preserve following program orders.
LOAD-LOAD
LOAD-STORE