[PATCH] riscv: fix race when vmap stack overflow

[Andrea Parri]

> > > +     atomic_set_release(&spin_shadow_stack, 0);
> >
> > Have not really looked the details: should there be a matching acquire?
> 
> I use atomic_set_release here, because I need earlier memory
> operations finished to make sure the sp is ready then set the spin
> flag.
> 
> The following memory operations order is not important, because we
> just care about sp value.
> 
> Also, we use relax amoswap before, because sp has naturelly
> dependency. But giving them RCsc is okay here, because we don't care
> about performance here.

Thanks for the clarification.

I'm not really suggesting to add unneeded synchronization, even more
so in local/private constructs as in this case.  It just felt odd to
see the release without a pairing acquire, so I asked.  ;-)

Thanks,
  Andrea


> eg:
>  handle_kernel_stack_overflow:
> +1:     la sp, spin_shadow_stack
> +       amoswap.w.aqrl sp, sp, (sp)
> +       bnez sp, 1b
> +
> ....
> +     smp_store_release(&spin_shadow_stack, 0);
> +     smp_mb();