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

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Date:	Mon, 3 Aug 2015 11:51:15 +0000
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	lkml <linux-kernel@...r.kernel.org>,
	"arc-linux-dev@...opsys.com" <arc-linux-dev@...opsys.com>
Subject: Re: [PATCH 4/6] ARC: LLOCK/SCOND based rwlock

On Monday 03 August 2015 05:03 PM, Peter Zijlstra wrote:
> 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?

Right - that was my overly cautious initial implementation. I did switch the spin
unlock to regular ST after initial experimenting, but missed this one. I'll take
it out in next version.

-Vineet
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