linux-kernel - Re: [PATCH v2 5/9] atomic,x86: Implement atomic_dec_and_test

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Date:   Mon, 13 Dec 2021 11:04:27 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     will@...nel.org, boqun.feng@...il.com,
        linux-kernel@...r.kernel.org, x86@...nel.org, elver@...gle.com,
        keescook@...omium.org, hch@...radead.org,
        torvalds@...ux-foundation.org, axboe@...nel.dk
Subject: Re: [PATCH v2 5/9] atomic,x86: Implement
 atomic_dec_and_test_overflow()

On Fri, Dec 10, 2021 at 05:16:23PM +0100, Peter Zijlstra wrote:
> Provide a better implementation of atomic_{dec,dec_and_test}_overflow() by
> making use of the atomic-op condition codes.
> 
> This further improves the fast path code:
> 
>     a980:       b8 ff ff ff ff          mov    $0xffffffff,%eax
>     a985:       f0 0f c1 07             lock xadd %eax,(%rdi)
>     a989:       83 e8 01                sub    $0x1,%eax
>     a98c:       78 20                   js     a9ae <ring_buffer_put+0x2e>
>     a98e:       74 01                   je     a991 <ring_buffer_put+0x11>
>     a990:       c3                      ret
> 
> to:
> 
>     a950:       f0 ff 0f                lock decl (%rdi)
>     a953:       7c 20                   jl     a975 <ring_buffer_put+0x25>
>     a955:       74 01                   je     a958 <ring_buffer_put+0x8>
>     a957:       c3                      ret

As a heads-up, for arm64 we probably won't need to roll our own since the LSE
atomics don't have flag-setting versions, and the compiler-generated
comparisons and conditional branches can be better optimized per caller.

Otherwise, I'm going to ignore this patch since I'm not qualified to review x86
asm. :)

Mark.

> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  arch/x86/include/asm/atomic.h |   23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
> 
> --- a/arch/x86/include/asm/atomic.h
> +++ b/arch/x86/include/asm/atomic.h
> @@ -263,6 +263,29 @@ static __always_inline int arch_atomic_f
>  }
>  #define arch_atomic_fetch_xor arch_atomic_fetch_xor
>  
> +#define arch_atomic_dec_overflow(_v, _label)				\
> +	asm_volatile_goto(LOCK_PREFIX "decl %[var]\n\t"			\
> +			  "jle %l1"					\
> +			  : : [var] "m" ((_v)->counter)			\
> +			  : "memory"					\
> +			  : _label)
> +
> +#define arch_atomic_dec_and_test_overflow(_v, _label)			\
> +({									\
> +	__label__ __zero;						\
> +	__label__ __out;						\
> +	bool __ret = false;						\
> +	asm_volatile_goto(LOCK_PREFIX "decl %[var]\n\t"			\
> +			  "jl %l2\n\t"					\
> +			  "je %l[__zero]"				\
> +			  : : [var] "m" ((_v)->counter)			\
> +			  : "memory"					\
> +			  : __zero, _label);				\
> +	goto __out;							\
> +__zero:	__ret = true;							\
> +__out:	__ret;								\
> +})
> +
>  #ifdef CONFIG_X86_32
>  # include <asm/atomic64_32.h>
>  #else
> 
>