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

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Message-ID: <87r1c6ad7e.fsf@mpe.ellerman.id.au>
Date:   Wed, 27 Oct 2021 23:01:41 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Peter Zijlstra <peterz@...radead.org>,
        Arnd Bergmann <arnd@...nel.org>
Cc:     Waiman Long <longman@...hat.com>, 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>,
        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

Peter Zijlstra <peterz@...radead.org> writes:
> On Mon, Oct 25, 2021 at 03:06:24PM +0200, 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.
>> 
>> On PowerPC, we pick at compile-time between either the qspinlock
>> (default-enabled on Book3S-64, i.e. all server chips) or a ll/sc based
>> spinlock plus vm_yield() (default on embedded and 32-bit mac).
>
> Urgh, yeah, so this crud undermines the whole point of having a fair
> lock. I'm thinking s390 and Power want to have this fixed.

Our Kconfig has:

  config GENERIC_LOCKBREAK
  	bool
  	default y
  	depends on SMP && PREEMPTION

And we have exactly one defconfig that enables both SMP and PREEMPT,
arch/powerpc/configs/85xx/ge_imp3a_defconfig, which is some ~10 year old
PCI card embedded thing I've never heard of. High chance anyone who has
those is not running upstream kernels on them.

So I think we'd be happy for you rip GENERIC_LOCKBREAK out, it's almost
entirely unused on powerpc anyway.

cheers