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

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Message-ID: <20150803130652.GO25159@twins.programming.kicks-ass.net>
Date:	Mon, 3 Aug 2015 15:06:52 +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" <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 01:02:09PM +0000, Vineet Gupta wrote:
> On Monday 03 August 2015 05:21 PM, Peter Zijlstra wrote:
> 
> 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.
> 
> 
> yeah !
> 
> - asl %[delay], %[delay], 1
> + asl.f  %[delay], %[delay], 1
> + mov.z %[delay], 1

The other obvious option is: 0x80000000 :-)
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