Re: [PATCH] locking: remove spin_lock_flags() etc
From: Waiman Long
Date: Mon Oct 25 2021 - 11:29:31 EST
On 10/25/21 9:06 AM, Arnd Bergmann wrote:
On Mon, Oct 25, 2021 at 11:57 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxxx> wrote:
On 10/22/21 7:59 AM, Arnd Bergmann wrote:
From: Arnd Bergmann <arnd@xxxxxxxx>
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@xxxxxxxx>
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