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

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Date:   Thu, 29 Nov 2018 13:04:18 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Bart Van Assche <bvanassche@....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 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.