linux-kernel - Re: [PATCH v2] atomics: fix atomic64_{read_acquire,set

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Date:   Fri, 4 Feb 2022 08:20:25 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-kernel@...r.kernel.org, ardb@...nel.org, arnd@...db.de,
        peterz@...radead.org, will@...nel.org
Subject: Re: [PATCH v2] atomics: fix atomic64_{read_acquire,set_release}
 fallbacks

On Thu, Feb 03, 2022 at 04:12:43PM +0000, Mark Rutland wrote:
> diff --git a/include/linux/atomic/atomic-arch-fallback.h b/include/linux/atomic/atomic-arch-fallback.h
> index a3dba31df01e..6db58d180866 100644
> --- a/include/linux/atomic/atomic-arch-fallback.h
> +++ b/include/linux/atomic/atomic-arch-fallback.h
> @@ -151,7 +151,16 @@
>  static __always_inline int
>  arch_atomic_read_acquire(const atomic_t *v)
>  {
> -	return smp_load_acquire(&(v)->counter);
> +	int ret;
> +
> +	if (__native_word(atomic_t)) {
> +		ret = smp_load_acquire(&(v)->counter);
> +	} else {
> +		ret = arch_atomic_read(v);
> +		__atomic_acquire_fence();
> +	}
> +
> +	return ret;
>  }
>  #define arch_atomic_read_acquire arch_atomic_read_acquire
>  #endif
> @@ -160,7 +169,12 @@ arch_atomic_read_acquire(const atomic_t *v)
>  static __always_inline void
>  arch_atomic_set_release(atomic_t *v, int i)
>  {
> -	smp_store_release(&(v)->counter, i);
> +	if (__native_word(atomic_t)) {
> +		smp_store_release(&(v)->counter, i);
> +	} else {
> +		__atomic_release_fence();
> +		arch_atomic_set(v, i);
> +	}
>  }

The changes for atomic_t are not necessary, right? They are correct but
"side effects" because of the change in scripts. If so, it's better to
mention this somewhere.

Regards,
Boqun

>  #define arch_atomic_set_release arch_atomic_set_release
>  #endif
> @@ -1258,7 +1272,16 @@ arch_atomic_dec_if_positive(atomic_t *v)
>  static __always_inline s64
>  arch_atomic64_read_acquire(const atomic64_t *v)
>  {
> -	return smp_load_acquire(&(v)->counter);
> +	s64 ret;
> +
> +	if (__native_word(atomic64_t)) {
> +		ret = smp_load_acquire(&(v)->counter);
> +	} else {
> +		ret = arch_atomic64_read(v);
> +		__atomic_acquire_fence();
> +	}
> +
> +	return ret;
>  }
>  #define arch_atomic64_read_acquire arch_atomic64_read_acquire
>  #endif
> @@ -1267,7 +1290,12 @@ arch_atomic64_read_acquire(const atomic64_t *v)
>  static __always_inline void
>  arch_atomic64_set_release(atomic64_t *v, s64 i)
>  {
> -	smp_store_release(&(v)->counter, i);
> +	if (__native_word(atomic64_t)) {
> +		smp_store_release(&(v)->counter, i);
> +	} else {
> +		__atomic_release_fence();
> +		arch_atomic64_set(v, i);
> +	}
>  }
>  #define arch_atomic64_set_release arch_atomic64_set_release
>  #endif
> @@ -2358,4 +2386,4 @@ arch_atomic64_dec_if_positive(atomic64_t *v)
>  #endif
>  
>  #endif /* _LINUX_ATOMIC_FALLBACK_H */
> -// cca554917d7ea73d5e3e7397dd70c484cad9b2c4
> +// 8e2cc06bc0d2c0967d2f8424762bd48555ee40ae
> diff --git a/scripts/atomic/fallbacks/read_acquire b/scripts/atomic/fallbacks/read_acquire
> index 803ba7561076..a0ea1d26e6b2 100755
> --- a/scripts/atomic/fallbacks/read_acquire
> +++ b/scripts/atomic/fallbacks/read_acquire
> @@ -2,6 +2,15 @@ cat <<EOF
>  static __always_inline ${ret}
>  arch_${atomic}_read_acquire(const ${atomic}_t *v)
>  {
> -	return smp_load_acquire(&(v)->counter);
> +	${int} ret;
> +
> +	if (__native_word(${atomic}_t)) {
> +		ret = smp_load_acquire(&(v)->counter);
> +	} else {
> +		ret = arch_${atomic}_read(v);
> +		__atomic_acquire_fence();
> +	}
> +
> +	return ret;
>  }
>  EOF
> diff --git a/scripts/atomic/fallbacks/set_release b/scripts/atomic/fallbacks/set_release
> index 86ede759f24e..05cdb7f42477 100755
> --- a/scripts/atomic/fallbacks/set_release
> +++ b/scripts/atomic/fallbacks/set_release
> @@ -2,6 +2,11 @@ cat <<EOF
>  static __always_inline void
>  arch_${atomic}_set_release(${atomic}_t *v, ${int} i)
>  {
> -	smp_store_release(&(v)->counter, i);
> +	if (__native_word(${atomic}_t)) {
> +		smp_store_release(&(v)->counter, i);
> +	} else {
> +		__atomic_release_fence();
> +		arch_${atomic}_set(v, i);
> +	}
>  }
>  EOF
> -- 
> 2.30.2
> 

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