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

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Message-ID: <20260123120302.TsiVMAQb@linutronix.de>
Date: Fri, 23 Jan 2026 13:03:02 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Harry Yoo <harry.yoo@...cle.com>, 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 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? 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…

> +	lock_map_acquire_try(&kfree_rcu_sheaf_map);
> +

Reviewed-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

Sebastian