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

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Date:   Tue, 7 Feb 2017 10:48:16 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Waiman Long <longman@...hat.com>, 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


* 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 Ingo, much of what this thing does seems to be superseded by both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