Peter Zijlstra [off-list ref] writes:

On Mon, Oct 25, 2021 at 03:06:24PM +0200, Arnd Bergmann wrote:

quoted

On Mon, Oct 25, 2021 at 11:57 AM Peter Zijlstra [off-list ref] wrote:

quoted

On Sat, Oct 23, 2021 at 06:04:57PM +0200, Arnd Bergmann wrote:

quoted

On Sat, Oct 23, 2021 at 3:37 AM Waiman Long [off-list ref] wrote:

quoted

quoted

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