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

Guo Ren
2022-07-04T11:10:34Z

On Mon, Jul 4, 2022 at 5:52 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Tue, Jun 28, 2022 at 04:17:03AM -0400, guoren@kernel.org wrote:
> > From: Guo Ren <guoren@linux.alibaba.com>
> >
> > 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@linux.alibaba.com>
> > Signed-off-by: Guo Ren <guoren@kernel.org>
> > Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
> > Cc: Arnd Bergmann <arnd@arndb.de>
> > Cc: Palmer Dabbelt <palmer@rivosinc.com>
> > ---
> > 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/