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

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Message-ID: <93c5b695-4c98-4b3d-99d7-592d949750be@roeck-us.net>
Date: Wed, 27 Nov 2024 16:08:26 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: Waiman Long <llong@...hat.com>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Andreas Larsson <andreas@...sler.com>, 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>
Subject: Re: [PATCH] sparc/pci: Make pci_poke_lock a raw_spinlock_t.

On 11/27/24 15:47, Waiman Long wrote:
> On 11/27/24 12:44 PM, Guenter Roeck wrote:
>> On 11/27/24 08:53, Sebastian Andrzej Siewior wrote:
>>> On 2024-11-27 08:02:50 [-0800], Guenter Roeck wrote:
>>>> On 11/27/24 07:39, Andreas Larsson wrote:
>>>>> Even though this is for sparc64, there is work being done looking into
>>>>> enabling RT for sparc32. If the amount of fixes needed to keep
>>>>> PROVE_RAW_LOCK_NESTING enabled is quite small at the moment I'd rather
>>>>> see it enabled for sparc rather than risking it becoming worse in the
>>>>> future.
>>>
>>> Okay. So you seem to be in favour of fixing the sparc64 splats Guenter
>>> reported?
>>>
>>>>> I don't know what the situation is for other architectures that does not
>>>>> support RT.
>>>>>
>>>>
>>>> For my part I still don't understand why PROVE_RAW_LOCK_NESTING is no longer
>>>> a configurable option, or in other words why it is mandated even for architectures
>>>> not supporting RT. To me this means that I'll either have to disable PROVE_LOCKING
>>>> for sparc or live with endless warning backtraces. The latter obscures real
>>>> problems, so it is a no-go.
>>>
>>> It is documented in Documentation/locking/locktypes.rst how the locks
>>> should nest. It is just nobody enabled it on sparc64 and tested. The
>>> option was meant temporary until the big read blocks are cleared.
>>>
>>
>> That doesn't explain why PROVE_RAW_LOCK_NESTING is now mandatory if
>> PROVE_LOCKING is enabled, even on architectures where is was not tested.
>> I am all for testing, but that doesn't include making it mandatory
>> even where it is known to fail. Enabling it by default, sure, no problem.
>> Dropping the option entirely after it is proven to no longer needed,
>> also no problem. But force-enabling it even where untested or, worse,
>> known to fail, is two steps too far.
> 
> The main reason for enforcing PROVE_RAW_LOCK_NESTING with PROVE_LOCKING is due to the fact that PREEMPT_RT kernel is much less tested than the non-RT kernel. I do agree that we shouldn't force this on arches that don't support PREEMPT_RT. However, once an arch decides to support PREEMPT_RT, they have to fix all these raw_spinlock nesting problems.
> 

config PROVE_RAW_LOCK_NESTING
-       bool
+       bool "Enable raw_spinlock - spinlock nesting checks" if ARCH_SUPPORTS_RT=n
         depends on PROVE_LOCKING
-       default y
+       default y if ARCH_SUPPORTS_RT

would have accomplished that while at the same time making it optional
for non-RT architectures.

Guenter