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

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Message-ID: <20170207084553.GW6500@twins.programming.kicks-ass.net>
Date:   Tue, 7 Feb 2017 09:45:53 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 0/3] locking/spinlock_debug: Change it to a mostly
 fair lock

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Ingo, much of what this thing does seems to be superseded by both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