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

Frederic Weisbecker
2025-10-31T16:08:12Z

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@kernel.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