Re: [PATCH v3 1/2] rtmutex: allow specifying a subclass for nested locking

[Joel Fernandes]

On Mon, May 28, 2018 at 09:17:51AM +0200, Peter Zijlstra wrote:
> On Sun, May 27, 2018 at 10:19:36PM -0700, Joel Fernandes wrote:
> 
> > > +static inline void __rt_mutex_lock(struct rt_mutex *lock, unsigned int subclass)
> > > +{
> > > +	might_sleep();
> > > +
> > > +	mutex_acquire(&lock->dep_map, subclass, 0, _RET_IP_);
> > > +	rt_mutex_fastlock(lock, TASK_UNINTERRUPTIBLE, rt_mutex_slowlock);
> > > +}
> > > +
> > > +#ifdef CONFIG_DEBUG_LOCK_ALLOC
> > > +/**
> > > + * rt_mutex_lock_nested - lock a rt_mutex
> > 
> > This ifdef seems consistent with other nested locking primitives, but its
> > kind of confusing.
> > 
> > The Kconfig.debug for DEBUG_LOCK_ALLOC says:
> > 
> > config DEBUG_LOCK_ALLOC
> > 	bool "Lock debugging: detect incorrect freeing of live locks"
> > 	[...]
> > 	help
> > 	 This feature will check whether any held lock (spinlock, rwlock,
> > 	 mutex or rwsem) is incorrectly freed by the kernel, via any of the
> > 	 memory-freeing routines (kfree(), kmem_cache_free(), free_pages(),
> > 	 vfree(), etc.), whether a live lock is incorrectly reinitialized via
> > 	 spin_lock_init()/mutex_init()/etc., or whether there is any lock
> > 	 held during task exit.
> > 
> > Shouldn't this ideally be ifdef'd under PROVE_LOCKING for this and other
> > locking primitives? Any idea what's the reason? I know PROVE_LOCKING selects
> > DEBUG_LOCK_ALLOC but still..
> 
> No, the reason is that DEBUG_LOCK_ALLOC needs the lockdep hooks to know
> which locks are held, so it can warn when we try and free a held one.
> PROVE_LOCKING builds upon that.
> 
> The the locking primitives should key off of DEBUG_LOCK_ALLOC for
> introducing the hooks.

Got it, thanks for the clarification Peter!

Regards,

 -Joel