linux-kernel - Re: [PATCH] locking/lockdep: Make lockdep_register_key() ignore 'debug

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Date:   Fri, 12 Apr 2019 07:47:19 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Bart Van Assche <bvanassche@....org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, Will Deacon <will.deacon@....com>,
        Waiman Long <longman@...hat.com>, shenghui <shhuiw@...mail.com>
Subject: Re: [PATCH] locking/lockdep: Make lockdep_register_key() ignore
 'debug_locks'


* Bart Van Assche <bvanassche@....org> wrote:

> If lockdep_register_key() and lockdep_unregister_key() are called with
> debug_locks == false then the following warning is reported:
> 
> WARNING: CPU: 2 PID: 15145 at kernel/locking/lockdep.c:4920 lockdep_unregister_key+0x1ad/0x240
> 
> That warning is reported because lockdep_unregister_key() ignores the
> value of 'debug_locks' and because the behavior of lockdep_register_key()
> depends on whether or not 'debug_locks' is set. Fix this inconsistency
> by making lockdep_register_key() unconditionally register lock keys.
> 
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Waiman Long <longman@...hat.com>
> Cc: shenghui <shhuiw@...mail.com>
> Reported-by: shenghui <shhuiw@...mail.com>
> Fixes: a0b0fd53e1e6 ("locking/lockdep: Free lock classes that are no longer in use") # v5.1-rc1.
> Signed-off-by: Bart Van Assche <bvanassche@....org>
> ---
>  kernel/locking/lockdep.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index d2d65bbfae01..a228509b62f1 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -1027,15 +1027,16 @@ void lockdep_register_key(struct lock_class_key *key)
>  	hash_head = keyhashentry(key);
>  
>  	raw_local_irq_save(flags);
> -	if (!graph_lock())
> -		goto restore_irqs;
> +	arch_spin_lock(&lockdep_lock);
> +	current->lockdep_recursion = 1;
>  	hlist_for_each_entry_rcu(k, hash_head, hash_entry) {
>  		if (WARN_ON_ONCE(k == key))
>  			goto out_unlock;
>  	}
>  	hlist_add_head_rcu(&key->hash_entry, hash_head);
>  out_unlock:
> -	graph_unlock();
> +	current->lockdep_recursion = 0;
> +	arch_spin_unlock(&lockdep_lock);
>  restore_irqs:
>  	raw_local_irq_restore(flags);
>  }

So why don't we add a debug_locks test to lockdep_unregister_key() 
instead? The general principle to bring lockdep to a screeching halt when 
bugs are detected, ASAP.

Thanks,

	Ingo