Re: [PATCH] riscv: locks: introduce ticket-based spinlock implementation

[Guo Ren]

Thx Peter,

On Wed, Mar 24, 2021 at 7:09 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Wed, Mar 24, 2021 at 10:14:52AM +0000, guoren@xxxxxxxxxx wrote:
> > +static inline void arch_spin_lock(arch_spinlock_t *lock)
> > +{
> > +     arch_spinlock_t lockval;
> > +     u32 tmp;
> > +
> > +     asm volatile (
> > +             "1:     lr.w    %0, %2          \n"
> > +             "       mv      %1, %0          \n"
> > +             "       addw    %0, %0, %3      \n"
> > +             "       sc.w    %0, %0, %2      \n"
> > +             "       bnez    %0, 1b          \n"
> > +             : "=&r" (tmp), "=&r" (lockval), "+A" (lock->lock)
> > +             : "r" (1 << TICKET_NEXT)
> > +             : "memory");
> >
> > +     while (lockval.tickets.next != lockval.tickets.owner) {
> > +             /*
> > +              * FIXME - we need wfi/wfe here to prevent:
> > +              *  - cache line bouncing
> > +              *  - saving cpu pipeline in multi-harts-per-core
> > +              *    processor
> > +              */
> > +             lockval.tickets.owner = READ_ONCE(lock->tickets.owner);
> > +     }
> >
> > +     __atomic_acquire_fence();
> >  }
>
> > +static inline void arch_spin_unlock(arch_spinlock_t *lock)
> >  {
> > +     smp_store_release(&lock->tickets.owner, lock->tickets.owner + 1);
> > +     /* FIXME - we need ipi/sev here to notify above */
> >  }
>
> Urgh, are you saying your WFE requires an explicit SEV like on ARM ? The
Yes, I'm considering that kind of code.

> ARM64 model is far superious IMO, and then you can use
> smp_cond_load_acquire() in arch_spin_lock() and call it a day.
Great tip, thx. I'll follow that.

-- 
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/