linux-kernel - Re: [RFC PATCH 1/3] workqueue: Add an interface to taint workqueue lockdep with reclaim

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Message-ID: <aPgxy3lIqW1rUGsx@lstrano-desk.jf.intel.com>
Date: Tue, 21 Oct 2025 18:22:19 -0700
From: Matthew Brost <matthew.brost@...el.com>
To: Tejun Heo <tj@...nel.org>
CC: <intel-xe@...ts.freedesktop.org>, <dri-devel@...ts.freedesktop.org>,
	<linux-kernel@...r.kernel.org>, <jiangshanlai@...il.com>,
	<simona.vetter@...ll.ch>, <christian.koenig@....com>, <pstanner@...hat.com>,
	<dakr@...nel.org>
Subject: Re: [RFC PATCH 1/3] workqueue: Add an interface to taint workqueue
 lockdep with reclaim

On Tue, Oct 21, 2025 at 01:28:31PM -1000, Tejun Heo wrote:
> Hello,
> 
> On Tue, Oct 21, 2025 at 03:04:14PM -0700, Matthew Brost wrote:
> > > Hmm... would it make sense to tie this to WQ_MEM_RECLAIM - ie. enable it
> > > implicitly on workqueues w/ the flag set?
> > 
> > I had considered this, and for a while I thought WQ_MEM_RECLAIM already
> > did what I'm suggesting—especially since I’ve spotted bugs in drivers
> > where I would have expected lockdep to catch them.
> > 
> > In my opinion, this approach is better, but it has a broader kernel-wide
> > scope and could potentially break some things. My subsequent patches
> > will likely break one or two DRM drivers, so it might not be a concern
> > to fix everything that breaks across the kernel. It's up to you which
> > route we want to take here.
> 
> Yeah, it is bothersome that WQ_MEM_RECLAIM doesn't currently have a way to
> ensure compliance. I just didn't know about the lockdep mechanism. Can you

I agree this is the best route to ensure compliance.

> please update the patch so that WQ_MEM_RECLAIM implicitly enables the
> checking?
> 

Sure, but a bunch of things immediately break—including a convoluted
case in my driver. I can fix the kernel to the extent that my CI catches
issues, and fix any obvious cases through manual inspection. However,
I suspect that if we merge this, we'll be dealing with fallout
throughout a kernel RC cycle.

Matt

> Thanks.
> 
> -- 
> tejun