linux-kernel - Re: [PATCH 5/5] s390/uaccess: add cmpxchg_user

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Date:   Wed, 9 Nov 2022 23:24:24 +0100
From:   Heiko Carstens <hca@...ux.ibm.com>
To:     Janis Schoetterl-Glausch <scgl@...ux.ibm.com>
Cc:     Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Jonathan Corbet <corbet@....net>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-s390@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Sven Schnelle <svens@...ux.ibm.com>
Subject: Re: [PATCH 5/5] s390/uaccess: add cmpxchg_user_key()

On Wed, Nov 09, 2022 at 04:46:29PM +0100, Janis Schoetterl-Glausch wrote:
> On Wed, 2022-11-02 at 15:19 +0100, Heiko Carstens wrote:
> > +	case 1: {
> > +		unsigned int prev, tmp, shift;
> > +
> > +		shift = (3 ^ (address & 3)) << 3;
> > +		address ^= address & 3;
> > +		asm volatile(
> > +			"	spka	0(%[key])\n"
> > +			"	sacf	256\n"
> > +			"0:	l	%[prev],%[address]\n"
> > +			"1:	nr	%[prev],%[mask]\n"
> > +			"	lr	%[tmp],%[prev]\n"
> > +			"	or	%[prev],%[old]\n"
> > +			"	or	%[tmp],%[new]\n"
> > +			"2:	cs	%[prev],%[tmp],%[address]\n"
> > +			"3:	jnl	4f\n"
> > +			"	xr	%[tmp],%[prev]\n"
> > +			"	nr	%[tmp],%[mask]\n"
> 
> Are you only entertaining cosmetic changes to cmpxchg.h?

I fail to parse what you are trying to say. Please elaborate.

> The loop condition being imprecise seems non-ideal.

What exactly is imprecise?

> > +			  [key] "a" (key),
> 
> Why did you get rid of the << 4 shift?
> That's inconsistent with the other uaccess functions that take an access key.

That's not only inconsistent, but also a bug.
Thank you for pointing this out. Will be fixed.