linux-kernel - Re: [PATCH v3 10/25] openrisc: add spinlock implementation

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Message-ID: <20170222113709.GP6515@twins.programming.kicks-ass.net>
Date:   Wed, 22 Feb 2017 12:37:09 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Stafford Horne <shorne@...il.com>
Cc:     Jonas Bonn <jonas@...thpole.se>,
        Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
        linux@...ck-us.net, openrisc@...ts.librecores.org,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v3 10/25] openrisc: add spinlock implementation

On Wed, Feb 22, 2017 at 04:11:39AM +0900, Stafford Horne wrote:


> +static inline int arch_spin_trylock(arch_spinlock_t *lock)
> +{
> +	unsigned long contended, tmp;
> +	u32 slock;
> +
> +	/* contended = (lock->tickets.owner != lock->tickets.next) */
> +	__asm__ __volatile__(
> +		"1:	l.lwa	%0, 0(%3)	\n"
> +		"	l.srli	%1, %0, 16	\n"
> +		"	l.andi	%2, %0, 0xffff	\n"
> +		"	l.sfeq	%1, %2		\n"
> +		"	l.bnf	1f		\n"
> +		"	 l.ori	%1, r0, 1	\n"
> +		"	l.add	%0, %0, %4	\n"
> +		"	l.swa	0(%3), %0	\n"
> +		"	l.bnf	1b		\n"
> +		"	 l.ori	%1, r0, 0	\n"

#ifdef CONFIG_SMP
		"	l.sync			\n"
#endif

> +		"1:				\n"
> +		: "=&r" (slock), "=&r" (contended), "=&r" (tmp)
> +		: "r" (&lock->slock), "r" (1 << TICKET_SHIFT)
> +		: "cc", "memory");
> +

Then s/contended/acquired/, flip the bitset in the asm, and replace the
entire thing below with:

	return acquired;

> +	if (!contended) {
> +		smp_mb();
> +		return 1;
> +	} else {
> +		return 0;
> +	}
> +}