linux-kernel - Re: [PATCH 11/33] cpuset: Provide lockdep check for cpuset lock held

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Message-ID: <aQTe6X5XXSp8_3z5@localhost.localdomain>
Date: Fri, 31 Oct 2025 17:08:09 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: Chen Ridong <chenridong@...weicloud.com>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Michal Koutný <mkoutny@...e.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Danilo Krummrich <dakr@...nel.org>,
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	Eric Dumazet <edumazet@...gle.com>,
	Gabriele Monaco <gmonaco@...hat.com>,
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Subject: Re: [PATCH 11/33] cpuset: Provide lockdep check for cpuset lock held

Le Tue, Oct 14, 2025 at 09:29:25PM +0800, Chen Ridong a écrit :
> 
> 
> On 2025/10/14 4:31, Frederic Weisbecker wrote:
> > cpuset modifies partitions, including isolated, while holding the cpuset
> > mutex.
> > 
> > This means that holding the cpuset mutex is safe to synchronize against
> > housekeeping cpumask changes.
> > 
> > Provide a lockdep check to validate that.
> > 
> > Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> > ---
> >  include/linux/cpuset.h | 2 ++
> >  kernel/cgroup/cpuset.c | 7 +++++++
> >  2 files changed, 9 insertions(+)
> > 
> > diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
> > index 2ddb256187b5..051d36fec578 100644
> > --- a/include/linux/cpuset.h
> > +++ b/include/linux/cpuset.h
> > @@ -18,6 +18,8 @@
> >  #include <linux/mmu_context.h>
> >  #include <linux/jump_label.h>
> >  
> > +extern bool lockdep_is_cpuset_held(void);
> > +
> >  #ifdef CONFIG_CPUSETS
> >  
> >  /*
> > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> > index 8595f1eadf23..aa1ac7bcf2ea 100644
> > --- a/kernel/cgroup/cpuset.c
> > +++ b/kernel/cgroup/cpuset.c
> > @@ -279,6 +279,13 @@ void cpuset_full_unlock(void)
> >  	cpus_read_unlock();
> >  }
> >  
> > +#ifdef CONFIG_LOCKDEP
> > +bool lockdep_is_cpuset_held(void)
> > +{
> > +	return lockdep_is_held(&cpuset_mutex);
> > +}
> > +#endif
> > +
> >  static DEFINE_SPINLOCK(callback_lock);
> >  
> >  void cpuset_callback_lock_irq(void)
> 
> Is the lockdep_is_cpuset_held function actually being used?
> If CONFIG_LOCKDEP is disabled, compilation would fail with an "undefined reference to
> lockdep_is_cpuset_held" error.

Although counter-intuitive, this is how the lockdep_is_held() functions family
do work.

This allows this kind of trick:

if (IS_ENABLED(CONFIG_LOCKDEP))
   WARN_ON_ONCE(!lockdep_is_held(&some_lock))

This works during the compilation because the prototype of lockdep_is_held()
is declared. And since the IS_ENABLED() is resolved during compilation as well,
the conditional code is wiped out and therefore not linked. As a result the
linker doesn't even look for the definition of lockdep_is_held() and we don't
need to define an off case that would return a wrong assumption.

Thanks.

-- 
Frederic Weisbecker
SUSE Labs