linux-kernel - Re: [PATCH 25/27] locking/lockdep: Add support for dynamic keys

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Message-ID: <1543510767.185366.145.camel@acm.org>
Date:   Thu, 29 Nov 2018 08:59:27 -0800
From:   Bart Van Assche <bvanassche@....org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, tj@...nel.org, johannes.berg@...el.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 25/27] locking/lockdep: Add support for dynamic keys

On Thu, 2018-11-29 at 13:04 +0100, Peter Zijlstra wrote:
> On Wed, Nov 28, 2018 at 03:43:23PM -0800, Bart Van Assche wrote:
> > A shortcoming of the current lockdep implementation is that it requires
> > lock keys to be allocated statically. That forces certain lock objects
> > to share lock keys. Since lock dependency analysis groups lock objects
> > per key sharing lock keys can cause false positive lockdep reports.
> > Make it possible to avoid such false positive reports by allowing lock
> > keys to be allocated dynamically. Require that dynamically allocated
> > lock keys are registered before use by calling lockdep_register_key().
> > Complain about attempts to register the same lock key pointer twice
> > without calling lockdep_unregister_key() between successive
> > registration calls.
> >  struct lock_class_key {
> > +	struct hlist_node		hash_entry;
> >  	struct lockdep_subclass_key	subkeys[MAX_LOCKDEP_SUBCLASSES];
> >  };
> 
> That hash_entry is purely for that double-register warning, right? I
> wonder if we can do that differently; by always doing
> register_lock_class(), and checking that state.

Hi Peter,

The hash_entry serves two purposes. One purpose is to verify whether the
lockdep_register_key() and  lockdep_unregister_key() functions are used
correctly. A second purpose is to avoid that lockdep_init_map() complains
when encountering a dynamically allocated key. I'm not sure how always
doing register_lock_class() would help?

Thanks,

Bart.