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

Waiman Long
2021-10-25T15:28:37Z

On 10/25/21 9:06 AM, 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.

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