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

[Matthew Brost]

On Tue, Oct 21, 2025 at 11:56:30AM -1000, Tejun Heo wrote:
> 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@xxxxxxxxxx>
> > Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
> > Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
> > ---
> >  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()?
> 

Can rename, but perhaps not needed depending on what we land on below.

> 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.

Matt 

> Thanks.
> 
> -- 
> tejun