From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v3 1/2] rtmutex: allow specifying a subclass for nested
 locking
Date: Mon, 28 May 2018 09:17:51 +0200
Message-ID: <20180528071751.GT12180@hirez.programming.kicks-ass.net>
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To: Joel Fernandes <joel@joelfernandes.org>
Cc: Peter Rosin <peda@axentia.se>, linux-kernel@vger.kernel.org, Wolfram Sang <wsa@the-dreams.de>, Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>, Davidlohr Bueso <dave@stgolabs.net>, Philippe Ombredanne <pombredanne@nexb.com>, Thomas Gleixner <tglx@linutronix.de>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, linux-i2c@vger.kernel.org, Peter Chang <dpf@google.com>, Deepa Dinamani <deepadinamani@google.com>, John Sperbeck <jsperbeck@google.com>
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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.