linux-kernel - Re: [PATCH v2 10/35] nds32: Atomic operations

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Message-ID: <20171127135724.yy2jjjuby3xu74mm@lakrids.cambridge.arm.com>
Date:   Mon, 27 Nov 2017 13:57:25 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Greentime Hu <green.hu@...il.com>
Cc:     greentime@...estech.com, linux-kernel@...r.kernel.org,
        arnd@...db.de, linux-arch@...r.kernel.org, tglx@...utronix.de,
        jason@...edaemon.net, marc.zyngier@....com, robh+dt@...nel.org,
        netdev@...r.kernel.org, deanbo422@...il.com,
        devicetree@...r.kernel.org, viro@...iv.linux.org.uk,
        dhowells@...hat.com, will.deacon@....com,
        daniel.lezcano@...aro.org, linux-serial@...r.kernel.org,
        Vincent Chen <vincentc@...estech.com>
Subject: Re: [PATCH v2 10/35] nds32: Atomic operations

Hi,

On Mon, Nov 27, 2017 at 08:27:57PM +0800, Greentime Hu wrote:
> +static inline void arch_spin_unlock(arch_spinlock_t * lock)
> +{
> +	asm volatile(
> +		"xor	$r15,  $r15, $r15\n"
> +		"swi	$r15, [%0]\n"
> +		:
> +		:"r"(&lock->lock)
> +		:"memory");
> +}

This looks suspicious. There's no clobber for $r15, so isn't this
corrupting a GPR behind the back of the compiler?

Shouldn't there be a tmp variable for the register allocation rather
than hard-coding $r15?

> +static inline void arch_write_unlock(arch_rwlock_t * rw)
> +{
> +	asm volatile(
> +		"xor	$r15, $r15, $r15\n"
> +		"swi	$r15, [%0]\n"
> +		:
> +		:"r"(&rw->lock)
> +		:"memory","$r15");
> +}

This time you have a clobber, but it's still not clear to me why you
don't use a tmp variable and leave the register allocation to the
compiler.

Thanks,
Mark.