linux-kernel - Re: [PATCH] sparc/pci: Make pci_poke_lock a raw_spinlock

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Message-ID: <55e2fcb8-dd06-42e4-b5de-4a0b46057571@roeck-us.net>
Date: Mon, 25 Nov 2024 13:54:14 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: Waiman Long <llong@...hat.com>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: sparclinux@...r.kernel.org, linux-kernel@...r.kernel.org,
 Boqun Feng <boqun.feng@...il.com>, Ingo Molnar <mingo@...hat.com>,
 Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>,
 Will Deacon <will@...nel.org>, "David S. Miller" <davem@...emloft.net>,
 Andreas Larsson <andreas@...sler.com>
Subject: Re: [PATCH] sparc/pci: Make pci_poke_lock a raw_spinlock_t.

On 11/25/24 13:29, Waiman Long wrote:
> 
> On 11/25/24 4:25 PM, Guenter Roeck wrote:
>> On 11/25/24 12:54, Waiman Long wrote:
>>>
>>> On 11/25/24 3:23 PM, Guenter Roeck wrote:
>>>> On 11/25/24 12:06, Guenter Roeck wrote:
>>>>> On 11/25/24 11:33, Waiman Long wrote:
>>>>> [ ... ]
>>>>>>> Fixing that finally gives me a clean run. Nevertheless, that makes me wonder:
>>>>>>> Should I just disable CONFIG_PROVE_RAW_LOCK_NESTING for sparc runtime tests ?
>>>>>>
>>>>>> If no one is tryng to ever enable PREEMPT_RT on SPARC, I suppose you could disable CONFIG_PROVE_RAW_LOCK_NESTING to avoid the trouble.
>>>>>>
>>>>>
>>>>> SGTM. I'll do that unless someone gives me a good reason to keep it enabled.
>>>>>
>>>>
>>>> Actually it can not be disabled with a configuration flag. It is
>>>> automatically enabled. I'll have to disable PROVE_LOCKING to disable it.
>>>>
>>>> config PROVE_RAW_LOCK_NESTING
>>>>         bool                    <---- no longer user configurable
>>>>         depends on PROVE_LOCKING
>>>>         default y
>>>>         help
>>>>          Enable the raw_spinlock vs. spinlock nesting checks which ensure
>>>>          that the lock nesting rules for PREEMPT_RT enabled kernels are
>>>>          not violated.
>>>>
>>>> I don't really like that, and I don't understand the logic behind it,
>>>> but it is what it is.
>>>>
>>>> FWIW, the description of commit 560af5dc839 is misleading. It says "Enable
>>>> PROVE_RAW_LOCK_NESTING _by default_" (emphasis mine). That is not what the
>>>> commit does. It force-enables PROVE_RAW_LOCK_NESTING if PROVE_LOCKING is
>>>> enabled. It is all or nothing.
>>>>
>>> I think we can relax it by
>>>
>>> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
>>> index 5d9eca035d47..bfdbd3fa2d29 100644
>>> --- a/lib/Kconfig.debug
>>> +++ b/lib/Kconfig.debug
>>> @@ -1399,7 +1399,7 @@ config PROVE_LOCKING
>>>   config PROVE_RAW_LOCK_NESTING
>>>          bool
>>>          depends on PROVE_LOCKING
>>> -       default y
>>> +       default y if ARCH_SUPPORTS_RT
>>>          help
>>>           Enable the raw_spinlock vs. spinlock nesting checks which ensure
>>>           that the lock nesting rules for PREEMPT_RT enabled kernels are
>>>
>>> Sebastian, what do you think?
>>>
>>
>>     depends on PROVE_LOCKING && ARCH_SUPPORTS_RT
>>
>> seems to make more sense to me.
> 
> That will work too, but that will enforce that arches with no ARCH_SUPPORTS_RT will not be able to enable PROVE_RAW_LOCK_NESTING even if people want to try it out.
> 

No architecture will be able to enable anything because "bool" has no
string associated with it. As mentioned before, it is all or nothing.
Otherwise I could just configure "CONFIG_PROVE_RAW_LOCK_NESTING=n"
for sparc and be done.

Guenter