Re: [PATCH] locking: remove spin_lock_flags() etc

From: Michael Ellerman <mpe@ellerman.id.au>
To: Peter Zijlstra <peterz@infradead.org>, Arnd Bergmann <arnd@kernel.org>
Cc: Waiman Long <longman@redhat.com>, Ingo Molnar <mingo@redhat.com>,
	Will Deacon <will@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
	Boqun Feng <boqun.feng@gmail.com>,
	Jonas Bonn <jonas@southpole.se>,
	Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>,
	Stafford Horne <shorne@gmail.com>,
	"James E.J. Bottomley" <James.Bottomley@hansenpartnership.com>,
	Helge Deller <deller@gmx.de>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Heiko Carstens <hca@linux.ibm.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Alexander Gordeev <agordeev@linux.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-ia64@vger.kernel.org,
	Openrisc <openrisc@lists.librecores.org>,
	Parisc List <linux-parisc@vger.kernel.org>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	linux-s390 <linux-s390@vger.kernel.org>
Subject: Re: [PATCH] locking: remove spin_lock_flags() etc
Date: Wed, 27 Oct 2021 23:01:41 +1100	[thread overview]
Message-ID: <87r1c6ad7e.fsf@mpe.ellerman.id.au> (raw)
In-Reply-To: <YXbAPIm47WwpYYup@hirez.programming.kicks-ass.net>

Peter Zijlstra <peterz@infradead.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@infradead.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@redhat.com> wrote:
>> > > >> On 10/22/21 7:59 AM, Arnd Bergmann wrote:
>> > > > > From: Arnd Bergmann <arnd@arndb.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@arndb.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