Re: [PATCH V2] asm-generic: ticket-lock: Optimize arch_spin_value_unlocked

[bibo mao]

在 2023/7/31 10:33, guoren@xxxxxxxxxx 写道:
> From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> 
> The arch_spin_value_unlocked would cause an unnecessary memory
> access to the contended value. Although it won't cause a significant
> performance gap in most architectures, the arch_spin_value_unlocked
> argument contains enough information. Thus, remove unnecessary
> atomic_read in arch_spin_value_unlocked().
> 
> The caller of arch_spin_value_unlocked() could benefit from this
> change. Currently, the only caller is lockref.
> 
> Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
> Cc: Waiman Long <longman@xxxxxxxxxx>
> Cc: David Laight <David.Laight@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> ---
> Changelog
> V2:
>  - Fixup commit log with Waiman advice.
>  - Add Waiman comment in the commit msg.
> ---
>  include/asm-generic/spinlock.h | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/include/asm-generic/spinlock.h b/include/asm-generic/spinlock.h
> index fdfebcb050f4..90803a826ba0 100644
> --- a/include/asm-generic/spinlock.h
> +++ b/include/asm-generic/spinlock.h
> @@ -68,11 +68,18 @@ static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
>  	smp_store_release(ptr, (u16)val + 1);
>  }
>  
> +static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
> +{
> +	u32 val = lock.counter;
> +
> +	return ((val >> 16) == (val & 0xffff));
> +}
I do not know much about lock, will it be cached in register without memory
access again like READ_ONCE or atomic_read?

Regards
Bibo Mao
> +
>  static __always_inline int arch_spin_is_locked(arch_spinlock_t *lock)
>  {
> -	u32 val = atomic_read(lock);
> +	arch_spinlock_t val = READ_ONCE(*lock);
>  
> -	return ((val >> 16) != (val & 0xffff));
> +	return !arch_spin_value_unlocked(val);
>  }
>  
>  static __always_inline int arch_spin_is_contended(arch_spinlock_t *lock)
> @@ -82,11 +89,6 @@ static __always_inline int arch_spin_is_contended(arch_spinlock_t *lock)
>  	return (s16)((val >> 16) - (val & 0xffff)) > 1;
>  }
>  
> -static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
> -{
> -	return !arch_spin_is_locked(&lock);
> -}
> -
>  #include <asm/qrwlock.h>
>  
>  #endif /* __ASM_GENERIC_SPINLOCK_H */