linux-kernel - Re: [PATCH v3 0/3] locking/spinlock_debug: Change it to a mostly fair lock

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Message-ID: <bf9e1850-9722-bebf-069d-e27caff0c090@redhat.com>
Date:   Tue, 7 Feb 2017 14:46:31 -0500
From:   Waiman Long <longman@...hat.com>
To:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Paul E. McKenney" <paulmck@...ibm.com>
Subject: Re: [PATCH v3 0/3] locking/spinlock_debug: Change it to a mostly fair
 lock

On 02/07/2017 04:48 AM, Ingo Molnar wrote:
> * Peter Zijlstra <peterz@...radead.org> wrote:
>
>> On Thu, Feb 02, 2017 at 09:15:26AM -0500, Waiman Long wrote:
>>>  v2->v3:
>>>   - Keep the original v1 patches but move patch 3 of v2 in front so
>>>     as to disable GENERIC_LOCKBREAK when DEBUG_LOCK_ALLOC is on.
>>>
>>>  v1->v2:
>>>   - Pack lockup and break_lock into a single 4-byte slot so as not
>>>     to in increase spinlock size when GENERIC_LOCKBREAK is
>>>     on. Hopefully that will be enough to fix a frame size too large
>>>     warning in 0-day build.
>>>   - Add a new patch to disable GENERIC_LOCKBREAK when DEBUG_LOCK_ALLOC
>>>     is on.
>>>
>>> The current debug spinlock implementation is a TATAS unfair lock. This
>>> can occasionally lead to system lockup with a debug kernel because
>>> of the unfairness of the lock rather than inherent locking problem.
>>>
>>> This patch set changes the debug spinlock implementation to a
>>> mostly fair spinlock based on the MCS lock similar to what is done
>>> in qspinlock. It also includes a patch that disable GENERIC_LOCKBREAK
>>> when DEBUG_LOCK_ALLOC is on.
>>
>> An alternative is to just delete the thing entirely..
>>
>> Ingo, much of what this thing does seems to be superseded by both
>> lockdep and a reliable NMI watchdog. Is there still value in
>> spinlock_debug?
> So there's still early stages when the NMI watchdog is not running, and 
> spinlock-debug can detect lockups in raw locks that lockdep does not cover, right?
>
> But yeah ... it would simplify things all around, so I'm not unsympathetic to the 
> idea...
>
> I've Cc:-ed a few other locking gents.
>
> Thanks,
>
> 	Ingo

I have no problem deleting the debug_spinlock code entirely. I can
update the patch to delete the code if you guys think that is the right
thing to do.

Cheers,
Longman