linux-kernel - Re: [PATCH v1 1/1] s390: mm: convert pgste locking functions to C

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Date:   Wed, 6 Dec 2023 10:15:41 +0100
From:   Heiko Carstens <hca@...ux.ibm.com>
To:     Alexander Gordeev <agordeev@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        borntraeger@...ibm.com, frankja@...ux.ibm.com, nrb@...ux.ibm.com,
        nsg@...ux.ibm.com, svens@...ux.ibm.com, gor@...ux.ibm.com,
        gerald.schaefer@...ux.ibm.com
Subject: Re: [PATCH v1 1/1] s390: mm: convert pgste locking functions to C

On Wed, Dec 06, 2023 at 10:01:18AM +0100, Alexander Gordeev wrote:
> On Tue, Dec 05, 2023 at 06:32:52PM +0100, Claudio Imbrenda wrote:
> ...
> > +	do {
> > +		value = __atomic64_or_barrier(PGSTE_PCL_BIT, ptr);
> 
> Would it make sense to cpu_relax() here, e.g with a follow-up patch?

No, because cpu_relax() is a no-op on our architecture (besides that it
translates to barrier(); but __atomic64_or_barrier() obviously comes also
with barrier() semantics).

We used to do diag 0x44 with cpu_relax() but that caused many performance
problems, therefore we removed diag 0x44 completely from the kernel quite
some time ago.

See also commit 1b68ac8678a8 ("s390: remove last diag 0x44 caller").