linux-kernel - Re: [PATCH v4 02/22] mm/slab: fix false lockdep warning in __kfree_rcu

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Message-ID: <aXSl7V7wyiqP5dPB@hyeyoo>
Date: Sat, 24 Jan 2026 19:58:53 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Vlastimil Babka <vbabka@...e.cz>, Petr Tesarik <ptesarik@...e.com>,
        Christoph Lameter <cl@...two.org>,
        David Rientjes <rientjes@...gle.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>, Hao Li <hao.li@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Uladzislau Rezki <urezki@...il.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev,
        bpf@...r.kernel.org, kasan-dev@...glegroups.com,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH v4 02/22] mm/slab: fix false lockdep warning in
 __kfree_rcu_sheaf()

On Fri, Jan 23, 2026 at 01:03:02PM +0100, Sebastian Andrzej Siewior wrote:
> On 2026-01-23 07:52:40 [+0100], Vlastimil Babka wrote:
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -6268,11 +6268,26 @@ static void rcu_free_sheaf(struct rcu_head *head)
> …
> > +static DEFINE_WAIT_OVERRIDE_MAP(kfree_rcu_sheaf_map, LD_WAIT_CONFIG);
> > +
> >  bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj)
> >  {
> >  	struct slub_percpu_sheaves *pcs;
> >  	struct slab_sheaf *rcu_sheaf;
> 
> Would it work to have here something like
> 	BUG_ON(IS_ENABLED(CONFIG_PREEMPT_RT));
> 
> or WARN_ON+return?

I think adding WARN_ON_ONCE() + return would be good enough.
Could you please adjust it, Vlastimil?

> The way the code is now it relies on the check in
> kvfree_call_rcu() and tells lockdep to be quiet. And since it gets
> optimized away…

Yeah, it makes sense to add a warning to avoid future mistake.

> > +	lock_map_acquire_try(&kfree_rcu_sheaf_map);
> > +
> 
> Reviewed-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

Thanks a lot!

-- 
Cheers,
Harry / Hyeonggon