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Date: Tue, 7 Feb 2017 11:53:08 -0800 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Waiman Long <longman@...hat.com> Cc: Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 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> Subject: Re: [PATCH v3 0/3] locking/spinlock_debug: Change it to a mostly fair lock On Tue, Feb 07, 2017 at 02:46:31PM -0500, Waiman Long wrote: > 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 Ingo, much of what this thing does seems to be superseded by both 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 My usual question is "how often does the spinlock_debug code find a problem that would be hard to find otherwise?" Probably unanswerable given the nature of Linux-kernel development, but I figured I would ask anyway. ;-) Thanx, Paul
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