netdev - Re: [PATCH bpf-next 2/3] tools, perf: use smp_{rmb,mb} barriers instead of {rmb,mb}

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Message-ID: <20181018153307.ayvmq6du3gnsyvro@ast-mbp.dhcp.thefacebook.com>
Date:   Thu, 18 Oct 2018 08:33:09 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Peter Zijlstra <peterz@...radead.org>, paulmck@...ux.vnet.ibm.com,
        will.deacon@....com, acme@...hat.com, yhs@...com,
        john.fastabend@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next 2/3] tools, perf: use smp_{rmb,mb} barriers
 instead of {rmb,mb}

On Thu, Oct 18, 2018 at 05:04:34PM +0200, Daniel Borkmann wrote:
>  #endif /* _TOOLS_LINUX_ASM_IA64_BARRIER_H */
> diff --git a/tools/arch/powerpc/include/asm/barrier.h b/tools/arch/powerpc/include/asm/barrier.h
> index a634da0..905a2c6 100644
> --- a/tools/arch/powerpc/include/asm/barrier.h
> +++ b/tools/arch/powerpc/include/asm/barrier.h
> @@ -27,4 +27,20 @@
>  #define rmb()  __asm__ __volatile__ ("sync" : : : "memory")
>  #define wmb()  __asm__ __volatile__ ("sync" : : : "memory")
> 
> +#if defined(__powerpc64__)
> +#define smp_lwsync()	__asm__ __volatile__ ("lwsync" : : : "memory")
> +
> +#define smp_store_release(p, v)			\
> +do {						\
> +	smp_lwsync();				\
> +	WRITE_ONCE(*p, v);			\
> +} while (0)
> +
> +#define smp_load_acquire(p)			\
> +({						\
> +	typeof(*p) ___p1 = READ_ONCE(*p);	\
> +	smp_lwsync();				\
> +	___p1;					\

I don't like this proliferation of asm.
Why do we think that we can do better job than compiler?
can we please use gcc builtins instead?
https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html
__atomic_load_n(ptr, __ATOMIC_ACQUIRE);
__atomic_store_n(ptr, val, __ATOMIC_RELEASE);
are done specifically for this use case if I'm not mistaken.
I think it pays to learn what compiler provides.