Re: [PATCH v6 13/13] riscv: Add qspinlock support

[Guo Ren]

On Tue, Nov 12, 2024 at 12:43 AM Will Deacon <will@xxxxxxxxxx> wrote:
>
> On Sun, Nov 03, 2024 at 03:51:53PM +0100, Alexandre Ghiti wrote:
> > In order to produce a generic kernel, a user can select
> > CONFIG_COMBO_SPINLOCKS which will fallback at runtime to the ticket
> > spinlock implementation if Zabha or Ziccrse are not present.
> >
> > Note that we can't use alternatives here because the discovery of
> > extensions is done too late and we need to start with the qspinlock
> > implementation because the ticket spinlock implementation would pollute
> > the spinlock value, so let's use static keys.
>
> I think the static key toggling takes a mutex (jump_label_lock()) which
> can take a spinlock (lock->wait_lock) internally, so I don't grok how
> this works:
>
> > +static void __init riscv_spinlock_init(void)
> > +{
> > +     char *using_ext = NULL;
> > +
> > +     if (IS_ENABLED(CONFIG_RISCV_TICKET_SPINLOCKS)) {
> > +             pr_info("Ticket spinlock: enabled\n");
> > +             return;
> > +     }
> > +
> > +     if (IS_ENABLED(CONFIG_RISCV_ISA_ZABHA) &&
> > +         IS_ENABLED(CONFIG_RISCV_ISA_ZACAS) &&
> > +         riscv_isa_extension_available(NULL, ZABHA) &&
> > +         riscv_isa_extension_available(NULL, ZACAS)) {
> > +             using_ext = "using Zabha";
> > +     } else if (riscv_isa_extension_available(NULL, ZICCRSE)) {
> > +             using_ext = "using Ziccrse";
> > +     }
> > +#if defined(CONFIG_RISCV_COMBO_SPINLOCKS)
> > +     else {
> > +             static_branch_disable(&qspinlock_key);
> > +             pr_info("Ticket spinlock: enabled\n");
> > +             return;
> > +     }
> > +#endif
>
> i.e. we've potentially already used the qspinlock at this point.
Yes, I've used qspinlock here. But riscv_spinlock_init is called with
irq_disabled and smp_off. That means this qspinlock only performs a
test-set lock behavior by qspinlock fast-path.

The qspinlock is a clean implementation. After qspin_unlock, the lock
value remains at zero, but the ticket lock makes the value dirty. So
we use Qspinlock at first or change it to ticket-lock before irq & smp
up.

>
> Will



-- 
Best Regards
 Guo Ren