[LLVMdev] dmb ishld in AArch64

[Taesoo Kim]

What is dmb ishst? hehe .. But tackling these problems one-by-one do
not seem the quickest way to test our idea; try -O0?

> static inline void hlist_add_before_rcu(struct hlist_node *n,
>                     struct hlist_node *next)
> {
>     n->pprev = next->pprev;
>     n->next = next;

     barrier();

>     rcu_assign_pointer(hlist_pprev_rcu(n), n);

     barrier();

>     next->pprev = &n->next;
> }
> 
> can reordered, and causes kernel crash.
> 
> f94006a8  ldr x8, [x21,#8]
> f9000275  str x21, [x19]
> d5033abf  dmb ishst
> f9400669  ldr x9, [x19,#8]
> f9000668  str x8, [x19,#8] <==== reordered str
> f9000133  str x19, [x9]
> f90006b3  str x19, [x21,#8]
> 
> It should be:
> 
> f94006a8  ldr x8, [x21,#8]
> f9000668  str x8, [x19,#8]
> f9000275  str x21, [x19]
> d5033abf  dmb ishst
> f9400669  ldr x9, [x19,#8]
> f9000133  str x19, [x9]
> f90006b3  str x19, [x21,#8]
> 
> I guess it’s because "dmb ishst" is not checked/tested? Any quick way to fix this?
> 
> Thanks,
> Chengyu