linux-kernel - Re: [PATCH V2 11/19] csky: Atomic operations

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Message-ID: <20180705180008.GG2530@hirez.programming.kicks-ass.net>
Date:   Thu, 5 Jul 2018 20:00:08 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Guo Ren <ren_guo@...ky.com>
Cc:     linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, daniel.lezcano@...aro.org,
        jason@...edaemon.net, arnd@...db.de, c-sky_gcc_upstream@...ky.com,
        gnu-csky@...tor.com, thomas.petazzoni@...tlin.com,
        wbx@...ibc-ng.org, green.hu@...il.com
Subject: Re: [PATCH V2 11/19] csky: Atomic operations

On Mon, Jul 02, 2018 at 01:30:14AM +0800, Guo Ren wrote:
> +#ifdef CONFIG_CPU_HAS_LDSTEX
> +ENTRY(csky_cmpxchg)
> +	USPTOKSP
> +	mfcr	a3, epc
> +	INCTRAP	a3
> +
> +	subi    sp, 8
> +	stw     a3, (sp, 0)
> +	mfcr    a3, epsr
> +	stw     a3, (sp, 4)
> +
> +	psrset	ee
> +1:
> +	ldex	a3, (a2)
> +	cmpne	a0, a3
> +	bt16	2f
> +	mov	a3, a1
> +	stex	a3, (a2)
> +	bez	a3, 1b
> +2:
> +	sync.is
> +	mvc	a0
> +	ldw	a3, (sp, 0)
> +	mtcr	a3, epc
> +	ldw     a3, (sp, 4)
> +	mtcr	a3, epsr
> +	addi	sp, 8
> +	KSPTOUSP
> +	rte
> +END(csky_cmpxchg)
> +#else

Please explain... if the CPU has LDEX/STEX, then _why_ do you need this?