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

[Wathsala Wathawana Vithanage]

> 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. 

> b) if yes, is there any difference in terms of performance between:
>      "ldr; dmb; ldr;"   vs "lda; ldr;"
>       ?
dmb is a full barrier, performance is poor.
I would assume (haven't measured) ldr; dmb; ldr to be less performant than lda;ldr;

> c) Comapring at 1) and 2) above, combination of
>    ldr [head]; dmb; lda [opposite_tail]:
>    looks     like an overkill to me.  Wouldn't just:
>    ldr [head]; dmb; ldr[opposite_tail];
>    be sufficient here?
lda [opposite_tail]: synchronizes with stlr in tail update that happens after array update.
So, it cannot be changed to ldr. 

lda can be replaced with ldapr (LDA with release consistency - processor consistency) 
which is more performant as lda is allowed to rise above stlr. Can be done with -mcpu=+rcpc

--wathsala