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Re: [PATCH 4/6] ARC: LLOCK/SCOND based rwlock

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On Mon, Aug 03, 2015 at 03:33:06PM +0530, Vineet Gupta wrote:
> With LLOCK/SCOND, the rwlock counter can be atomically updated w/o need
> for a guarding spin lock.

Maybe re-iterate the exclusive vs shared spin story again.

And aside from the far too many full barriers (again), I was just
wondering about:

> +static inline void arch_write_unlock(arch_rwlock_t *rw)
> +{
> +	unsigned int val;
> +
> +	smp_mb();
> +
> +	/*
> +	 * rw->counter = __ARCH_RW_LOCK_UNLOCKED__;
> +	 */
> +	__asm__ __volatile__(
> +	"1:	llock	%[val], [%[rwlock]]	\n"
> +	"	scond	%[UNLOCKED], [%[rwlock]]\n"
> +	"	bnz	1b			\n"
> +	"					\n"
> +	: [val]		"=&r"	(val)
> +	: [rwlock]	"r"	(&(rw->counter)),
> +	  [UNLOCKED]	"r"	(__ARCH_RW_LOCK_UNLOCKED__)
> +	: "memory", "cc");
> +
> +	smp_mb();
> +}

Why can't that be a straight store?
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[PATCH 0/6] ARC: spinlocks/atomics rework Vineet Gupta <Vineet.Gupta1@synopsys.com> - 2015-08-03 12:10 +0200
  [PATCH 4/6] ARC: LLOCK/SCOND based rwlock Vineet Gupta <Vineet.Gupta1@synopsys.com> - 2015-08-03 12:10 +0200
    Re: [PATCH 4/6] ARC: LLOCK/SCOND based rwlock Peter Zijlstra <peterz@infradead.org> - 2015-08-03 13:40 +0200
      Re: [PATCH 4/6] ARC: LLOCK/SCOND based rwlock Vineet Gupta <Vineet.Gupta1@synopsys.com> - 2015-08-03 14:00 +0200
  [PATCH 6/6] ARCv2: spinlock/rwlock: Reset retry delay when starting a new spin-wait cycle Vineet Gupta <Vineet.Gupta1@synopsys.com> - 2015-08-03 12:10 +0200
    Re: [PATCH 6/6] ARCv2: spinlock/rwlock: Reset retry delay when  starting a new spin-wait cycle Peter Zijlstra <peterz@infradead.org> - 2015-08-03 13:50 +0200
      Re: [PATCH 6/6] ARCv2: spinlock/rwlock: Reset retry delay when  starting a new spin-wait cycle Vineet Gupta <Vineet.Gupta1@synopsys.com> - 2015-08-03 16:50 +0200
        Re: [PATCH 6/6] ARCv2: spinlock/rwlock: Reset retry delay when  starting a new spin-wait cycle Peter Zijlstra <peterz@infradead.org> - 2015-08-03 16:50 +0200
  [PATCH 5/6] ARCv2: spinlock/rwlock/atomics: Delayed retry of failed SCOND with exponential backoff Vineet Gupta <Vineet.Gupta1@synopsys.com> - 2015-08-03 12:10 +0200
    Re: [PATCH 5/6] ARCv2: spinlock/rwlock/atomics: Delayed retry of  failed SCOND with exponential backoff Peter Zijlstra <peterz@infradead.org> - 2015-08-03 13:50 +0200
      Re: [PATCH 5/6] ARCv2: spinlock/rwlock/atomics: Delayed retry of  failed SCOND with exponential backoff Vineet Gupta <Vineet.Gupta1@synopsys.com> - 2015-08-03 15:10 +0200
        Re: [PATCH 5/6] ARCv2: spinlock/rwlock/atomics: Delayed retry of  failed SCOND with exponential backoff Vineet Gupta <Vineet.Gupta1@synopsys.com> - 2015-08-03 16:00 +0200
          Re: [PATCH 5/6] ARCv2: spinlock/rwlock/atomics: Delayed retry of  failed SCOND with exponential backoff Peter Zijlstra <peterz@infradead.org> - 2015-08-03 16:10 +0200
    Re: [PATCH 5/6] ARCv2: spinlock/rwlock/atomics: Delayed retry of  failed SCOND with exponential backoff Peter Zijlstra <peterz@infradead.org> - 2015-08-03 14:00 +0200
      Re: [PATCH 5/6] ARCv2: spinlock/rwlock/atomics: Delayed retry of  failed SCOND with exponential backoff Peter Zijlstra <peterz@infradead.org> - 2015-08-03 15:10 +0200
      Re: [PATCH 5/6] ARCv2: spinlock/rwlock/atomics: Delayed retry of  failed SCOND with exponential backoff Vineet Gupta <Vineet.Gupta1@synopsys.com> - 2015-08-03 15:10 +0200
  [PATCH 3/6] ARC: LLOCK/SCOND based spin_lock Vineet Gupta <Vineet.Gupta1@synopsys.com> - 2015-08-03 12:10 +0200
    Re: [PATCH 3/6] ARC: LLOCK/SCOND based spin_lock Peter Zijlstra <peterz@infradead.org> - 2015-08-03 13:30 +0200
      Re: [PATCH 3/6] ARC: LLOCK/SCOND based spin_lock Vineet Gupta <Vineet.Gupta1@synopsys.com> - 2015-08-03 13:50 +0200
  [PATCH 1/6] Revert "ARCv2: STAR 9000837815 workaround hardware exclusive transactions livelock" Vineet Gupta <Vineet.Gupta1@synopsys.com> - 2015-08-03 12:10 +0200
  [PATCH 2/6] ARC: refactor atomic inline asm operands with symbolic names Vineet Gupta <Vineet.Gupta1@synopsys.com> - 2015-08-03 12:10 +0200

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