Re: [PATCH V7 1/5] asm-generic: ticket-lock: Remove unnecessary atomic_read

[Guo Ren]

On Mon, Jul 4, 2022 at 5:52 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Tue, Jun 28, 2022 at 04:17:03AM -0400, guoren@xxxxxxxxxx wrote:
> > From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> >
> > Remove unnecessary atomic_read in arch_spin_value_unlocked(lock),
> > because the value has been in lock. This patch could prevent
> > arch_spin_value_unlocked contend spin_lock data again.
> >
> > Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
> > Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> > Cc: Arnd Bergmann <arnd@xxxxxxxx>
> > Cc: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
> > ---
> >  include/asm-generic/spinlock.h | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/asm-generic/spinlock.h b/include/asm-generic/spinlock.h
> > index fdfebcb050f4..f1e4fa100f5a 100644
> > --- a/include/asm-generic/spinlock.h
> > +++ b/include/asm-generic/spinlock.h
> > @@ -84,7 +84,9 @@ static __always_inline int arch_spin_is_contended(arch_spinlock_t *lock)
> >
> >  static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
> >  {
> > -     return !arch_spin_is_locked(&lock);
> > +     u32 val = lock.counter;
> > +
> > +     return ((val >> 16) == (val & 0xffff));
> >  }
>
> Wouldn't the right thing be to flip arch_spin_is_locked() and
> arch_spin_value_is_unlocked() ?
Okay, I agree with your patch. Next version, I would take the below code.

>
>
> diff --git a/include/asm-generic/spinlock.h b/include/asm-generic/spinlock.h
> index fdfebcb050f4..63ab4da262f2 100644
> --- a/include/asm-generic/spinlock.h
> +++ b/include/asm-generic/spinlock.h
> @@ -68,23 +68,25 @@ static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
>         smp_store_release(ptr, (u16)val + 1);
>  }
>
> -static __always_inline int arch_spin_is_locked(arch_spinlock_t *lock)
> +static __always_inline int arch_spin_is_contended(arch_spinlock_t *lock)
>  {
>         u32 val = atomic_read(lock);
>
> -       return ((val >> 16) != (val & 0xffff));
> +       return (s16)((val >> 16) - (val & 0xffff)) > 1;
>  }
>
> -static __always_inline int arch_spin_is_contended(arch_spinlock_t *lock)
> +static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
>  {
> -       u32 val = atomic_read(lock);
> +       u32 val = lock.counter;
>
> -       return (s16)((val >> 16) - (val & 0xffff)) > 1;
> +       return ((val >> 16) == (val & 0xffff));
>  }
>
> -static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
> +static __always_inline int arch_spin_is_locked(arch_spinlock_t *lock)
>  {
> -       return !arch_spin_is_locked(&lock);
> +       arch_spinlock_t val = READ_ONCE(*lock);
> +
> +       return !arch_spin_value_unlocked(val);
>  }
>
>  #include <asm/qrwlock.h>
>


-- 
Best Regards
 Guo Ren

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