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

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Message-ID: <aPgBjmIm6n9H-R_u@slm.duckdns.org>
Date: Tue, 21 Oct 2025 11:56:30 -1000
From: Tejun Heo <tj@...nel.org>
To: Matthew Brost <matthew.brost@...el.com>
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

Hello,

On Tue, Oct 21, 2025 at 02:39:50PM -0700, Matthew Brost wrote:
> Drivers often use workqueues that are in the reclaim path (e.g., DRM
> scheduler workqueues). It is useful to teach lockdep that memory cannot
> be allocated on these workqueues. Add an interface to taint workqueue
> lockdep with reclaim.

Given that it's about reclaim, "memory cannot be allocated" may be a bit
misleading. Can you make the description more accurate? Also, it'd be great
if you can include an example lockdep splat for reference.

> Cc: Tejun Heo <tj@...nel.org>
> Cc: Lai Jiangshan <jiangshanlai@...il.com>
> Signed-off-by: Matthew Brost <matthew.brost@...el.com>
> ---
>  include/linux/workqueue.h | 19 +++++++++++++++++++
>  kernel/workqueue.c        |  9 +++++++++
>  2 files changed, 28 insertions(+)
> 
> diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
> index dabc351cc127..954c7eb7e225 100644
> --- a/include/linux/workqueue.h
> +++ b/include/linux/workqueue.h
> @@ -553,6 +553,25 @@ alloc_workqueue_lockdep_map(const char *fmt, unsigned int flags, int max_active,
>  						1, lockdep_map, ##args))
>  #endif
>  
> +
> +#ifdef CONFIG_LOCKDEP
> +/**
> + * taint_reclaim_workqueue - taint workqueue lockdep map with reclaim
> + * @wq: workqueue to taint with reclaim
> + * gfp: gfp taint
      ^@

> + *
> + * Drivers often use workqueues that are in the reclaim path (e.g., DRM
> + * scheduler workqueues). It is useful to teach lockdep that memory cannot be
> + * allocated on these workqueues.
> + */
> +extern void taint_reclaim_workqueue(struct workqueue_struct *wq, gfp_t gfp);
> +#else
> +static inline void taint_reclaim_workqueue(struct workqueue_struct *wq,
> +					   gfp_t gfp)

Would a more direct name work better, maybe something like
workqueue_warn_on_reclaim()?

Hmm... would it make sense to tie this to WQ_MEM_RECLAIM - ie. enable it
implicitly on workqueues w/ the flag set?

Thanks.

-- 
tejun