[PATCH] riscv: fix race when vmap stack overflow

[Andrea Parri]

Hi Tong,
 
> > > 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.

> 	Consider this implementation:)
> 
> 	smp_store_mb(&spin_shadow_stack, 0);

smp_store_mb() has "WRITE_ONCE(); smp_mb()" semantics; so it doesn't
guarantee that the store to spin_shadow_stack is ordered after program
-order earlier memory accesses.

  Andrea