Re: [PATCH] block: switch to atomic_t for request references

From: Peter Zijlstra <peterz@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Christoph Hellwig <hch@infradead.org>,
	Jens Axboe <axboe@kernel.dk>,
	"linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
	Kees Cook <keescook@chromium.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] block: switch to atomic_t for request references
Date: Tue, 7 Dec 2021 18:41:05 +0100	[thread overview]
Message-ID: <Ya+csba8U3v3DMVo@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <Ya+RPbdgEn6l6RbS@hirez.programming.kicks-ass.net>

On Tue, Dec 07, 2021 at 05:52:13PM +0100, Peter Zijlstra wrote:
> It's a bit gross, and there seems to be a problem with macro expansion
> of __ofl, but it 'works'.
> 
> ---
> diff --git a/arch/x86/include/asm/atomic.h b/arch/x86/include/asm/atomic.h
> index 5e754e895767..921ecfd5a40b 100644
> --- a/arch/x86/include/asm/atomic.h
> +++ b/arch/x86/include/asm/atomic.h
> @@ -263,6 +263,22 @@ static __always_inline int arch_atomic_fetch_xor(int i, atomic_t *v)
>  }
>  #define arch_atomic_fetch_xor arch_atomic_fetch_xor
>  
> +#define atomic_dec_and_test_ofl(_v, __ofl)				\
> +({									\
> +	__label__ __zero;						\
> +	__label__ __out;						\
> +	bool __ret = false;						\
> +	asm_volatile_goto (LOCK_PREFIX "decl %[var]\n\t"		\
> +			   "jz %l[__zero]\n\t"				\
> +			   "jl %l[__ofl]"				\
				%l2

and it works much better...

> +			   : : [var] "m" ((_v)->counter)		\
> +			   : "memory"					\
> +			   : __zero, __ofl);				\
> +	goto __out;							\
> +__zero:	__ret = true;							\
> +__out:	__ret;								\
> +})