linux-kernel - Re: usage of DEFINE_WAIT_OVERRIDE_MAP(LD_WAIT

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Message-ID: <20251023152942.GC26461@redhat.com>
Date: Thu, 23 Oct 2025 17:29:43 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: "Luis Claudio R. Goncalves" <lgoncalv@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Clark Williams <clrkwllms@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>, Tejun Heo <tj@...nel.org>,
	David Vernet <dvernet@...a.com>, Barret Rhoden <brho@...gle.com>,
	Josh Don <joshdon@...gle.com>, Crystal Wood <crwood@...hat.com>,
	linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev,
	Juri Lelli <juri.lelli@...hat.com>, Ben Segall <bsegall@...gle.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Ingo Molnar <mingo@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Wander Lairson Costa <wander@...hat.com>
Subject: Re: usage of DEFINE_WAIT_OVERRIDE_MAP(LD_WAIT_SLEEP)

On 10/23, Sebastian Andrzej Siewior wrote:
>
> On 2025-10-20 16:53:10 [+0200], Oleg Nesterov wrote:
> >
> > If CONFIG_PREEMPT_RT=y and preemptible() is true, LD_WAIT_CONFIG won't work.
> > lockdep will complain if fill_pool() takes a spinlock_t, spinlock is
> > LD_WAIT_SLEEP on RT.
>
> Where is this? This is then wrong then because _SLEEP is for MUTEX and
> those things that sleep-sleep and pretend to sleep.
>
> spinlock_t should be LD_WAIT_CONFIG. This is used in
> include/linux/spinlock_types.h via SPIN_DEP_MAP_INIT which is only
> defined once as LD_WAIT_CONFIG.

Hmm. I'll recheck, quite possibly I missed something.

But. From include/linux/spinlock_types.h

	/* PREEMPT_RT kernels map spinlock to rt_mutex */
	#include <linux/rtmutex.h>

	typedef struct spinlock {
		struct rt_mutex_base	lock;
	#ifdef CONFIG_DEBUG_LOCK_ALLOC
		struct lockdep_map	dep_map;
	#endif
	} spinlock_t;

> rt_mutex is defined as LD_WAIT_SLEEP but this one is not the one we use
> for spinlock_t.

see above...

> What you want to have working in the end is the following scenario:
>
> |static DEFINE_SPINLOCK(lock_spinlock);
> |static DEFINE_RAW_SPINLOCK(lock_raw_spinlock);
> |static DEFINE_RAW_SPINLOCK(lock_raw_spinlock2);
> |
> |static void lockdep_test(void)
> |{
> |        static DEFINE_WAIT_OVERRIDE_MAP(put_task_map, LD_WAIT_CONFIG);
> |
> |        raw_spin_lock(&lock_raw_spinlock);
> |        lock_map_acquire_try(&put_task_map);
> |
> |        spin_lock(&lock_spinlock);

Yes, but this won't work on RT? Again, I didn't check (yet), possibly I am
wrong...

> This will work. On RT as well but is wrong and will not happen due to
> the preemptible() check. On !RT it will so you want lockdep to be quiet.

See above and please correct me.

Oleg.