linux-kernel - Re: [PATCH 5/6] ARCv2: spinlock/rwlock/atomics: Delayed retry of failed SCOND with exponential backoff

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Date:	Mon, 3 Aug 2015 13:50:59 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:	lkml <linux-kernel@...r.kernel.org>, arc-linux-dev@...opsys.com
Subject: Re: [PATCH 5/6] ARCv2: spinlock/rwlock/atomics: Delayed retry of
 failed SCOND with exponential backoff

On Mon, Aug 03, 2015 at 03:33:07PM +0530, Vineet Gupta wrote:
> +#define SCOND_FAIL_RETRY_ASM							\
> +	"	bz	4f			\n"				\
> +	"   ; --- scond fail delay ---		\n"				\
> +	"	mov	%[tmp], %[delay]	\n"	/* tmp = delay */	\
> +	"2: 	brne.d	%[tmp], 0, 2b		\n"	/* while (tmp != 0) */	\
> +	"	sub	%[tmp], %[tmp], 1	\n"	/* tmp-- */		\
> +	"	asl	%[delay], %[delay], 1	\n"	/* delay *= 2 */	\
> +	"	b	1b			\n"	/* start over */	\
> +	"4: ; --- success ---			\n"				\

One more note, you might want a test to handle the case where delay *= 2
overflows and results in a 0.
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