linux-kernel - Re: [PATCH] locking: remove spin_lock

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Message-ID: <2413f412-a390-bbc0-e848-e2a77d1f0ab3@redhat.com>
Date:   Mon, 25 Oct 2021 11:28:37 -0400
From:   Waiman Long <longman@...hat.com>
To:     Arnd Bergmann <arnd@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Boqun Feng <boqun.feng@...il.com>,
        Jonas Bonn <jonas@...thpole.se>,
        Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
        Stafford Horne <shorne@...il.com>,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-ia64@...r.kernel.org,
        Openrisc <openrisc@...ts.librecores.org>,
        Parisc List <linux-parisc@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-s390 <linux-s390@...r.kernel.org>
Subject: Re: [PATCH] locking: remove spin_lock_flags() etc


On 10/25/21 9:06 AM, Arnd Bergmann wrote:
> On Mon, Oct 25, 2021 at 11:57 AM Peter Zijlstra <peterz@...radead.org> wrote:
>> On Sat, Oct 23, 2021 at 06:04:57PM +0200, Arnd Bergmann wrote:
>>> On Sat, Oct 23, 2021 at 3:37 AM Waiman Long <longman@...hat.com> wrote:
>>>>> On 10/22/21 7:59 AM, Arnd Bergmann wrote:
>>>>> From: Arnd Bergmann <arnd@...db.de>
>>>>>
>>>>> As this is all dead code, just remove it and the helper functions built
>>>>> around it. For arch/ia64, the inline asm could be cleaned up, but
>>>>> it seems safer to leave it untouched.
>>>>>
>>>>> Signed-off-by: Arnd Bergmann <arnd@...db.de>
>>>> Does that mean we can also remove the GENERIC_LOCKBREAK config option
>>>> from the Kconfig files as well?
>>>   I couldn't figure this out.
>>>
>>> What I see is that the only architectures setting GENERIC_LOCKBREAK are
>>> nds32, parisc, powerpc, s390, sh and sparc64, while the only architectures
>>> implementing arch_spin_is_contended() are arm32, csky and ia64.
>>>
>>> The part I don't understand is whether the option actually does anything
>>> useful any more after commit d89c70356acf ("locking/core: Remove break_lock
>>> field when CONFIG_GENERIC_LOCKBREAK=y").
>> Urgh, what a mess.. AFAICT there's still code in
>> kernel/locking/spinlock.c that relies on it. Specifically when
>> GENERIC_LOCKBREAK=y we seem to create _lock*() variants that are
>> basically TaS locks which drop preempt/irq disable while spinning.
>>
>> Anybody having this on and not having native TaS locks is in for a rude
>> surprise I suppose... sparc64 being the obvious candidate there :/
> Is this a problem on s390 and powerpc, those two being the ones
> that matter in practice?
>
> On s390, we pick between the cmpxchg() based directed-yield when
> running on virtualized CPUs, and a normal qspinlock when running on a
> dedicated CPU.

I am not aware that s390 is using qspinlocks at all as I don't see 
ARCH_USE_QUEUED_SPINLOCKS being set anywhere under arch/s390. I only see 
that it uses a cmpxchg based spinlock.

Cheers,
Longman