Re: [PATCH v3] Avoid memory barrier in read_seqcount() through load acquire

[Will Deacon]

Hi Christoph,

On Thu, Sep 12, 2024 at 03:44:08PM -0700, Christoph Lameter via B4 Relay wrote:
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 975dd22a2dbd..3c270f496231 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -1600,6 +1600,14 @@ config ARCH_HAS_KERNEL_FPU_SUPPORT
>  	  Architectures that select this option can run floating-point code in
>  	  the kernel, as described in Documentation/core-api/floating-point.rst.
>  
> +config ARCH_HAS_ACQUIRE_RELEASE
> +	bool
> +	help
> +	  Setting ARCH_HAS_ACQUIRE_RELEASE indicates that the architecture
> +	  supports load acquire and release. Typically these are more effective
> +	  than memory barriers. Code will prefer the use of load acquire and
> +	  store release over memory barriers if this option is enabled.
> +

Unsurprisingly, I'd be in favour of making this unconditional rather than
adding a new Kconfig option. Would that actually hurt any architectures
where we care about the last few shreds of performance?

>  source "kernel/gcov/Kconfig"
>  
>  source "scripts/gcc-plugins/Kconfig"
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index a2f8ff354ca6..19e34fff145f 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -39,6 +39,7 @@ config ARM64
>  	select ARCH_HAS_PTE_DEVMAP
>  	select ARCH_HAS_PTE_SPECIAL
>  	select ARCH_HAS_HW_PTE_YOUNG
> +	select ARCH_HAS_ACQUIRE_RELEASE
>  	select ARCH_HAS_SETUP_DMA_OPS
>  	select ARCH_HAS_SET_DIRECT_MAP
>  	select ARCH_HAS_SET_MEMORY
> diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
> index d90d8ee29d81..a3fe9ee8edef 100644
> --- a/include/linux/seqlock.h
> +++ b/include/linux/seqlock.h
> @@ -23,6 +23,13 @@
>  
>  #include <asm/processor.h>
>  
> +#ifdef CONFIG_ARCH_HAS_ACQUIRE_RELEASE
> +# define USE_LOAD_ACQUIRE	true
> +# define USE_COND_LOAD_ACQUIRE	!IS_ENABLED(CONFIG_PREEMPT_RT)
> +#else
> +# define USE_LOAD_ACQUIRE	false
> +# define USE_COND_LOAD_ACQUIRE	false
> +#endif
>  /*
>   * The seqlock seqcount_t interface does not prescribe a precise sequence of
>   * read begin/retry/end. For readers, typically there is a call to
> @@ -132,6 +139,17 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
>  #define seqcount_rwlock_init(s, lock)		seqcount_LOCKNAME_init(s, lock, rwlock)
>  #define seqcount_mutex_init(s, lock)		seqcount_LOCKNAME_init(s, lock, mutex)
>  
> +static __always_inline unsigned __seqprop_load_sequence(const seqcount_t *s, bool acquire)
> +{
> +	if (!acquire || !USE_LOAD_ACQUIRE)
> +		return READ_ONCE(s->sequence);
> +
> +	if (USE_COND_LOAD_ACQUIRE)
> +		return smp_cond_load_acquire((unsigned int *)&s->sequence, (s->sequence & 1) == 0);

This looks wrong to me.

The conditional expression passed to smp_cond_load_acquire() should be
written in terms of 'VAL', otherwise you're introducing an additional
non-atomic access to the sequence counter.

Will