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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
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