linux-kernel - Re: [PATCH] locking/lockdep: Zap lock classes even with lock debugging disabled

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Message-ID: <20190403124410.GE16362@fuggles.cambridge.arm.com>
Date:   Wed, 3 Apr 2019 13:44:10 +0100
From:   Will Deacon <will.deacon@....com>
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, Waiman Long <longman@...hat.com>,
        shenghui <shhuiw@...mail.com>
Subject: Re: [PATCH] locking/lockdep: Zap lock classes even with lock
 debugging disabled

On Tue, Mar 26, 2019 at 12:59:12PM -0700, Bart Van Assche wrote:
> Commit a0b0fd53e1e6 ("locking/lockdep: Free lock classes that are no longer
> in use") changed the behavior of lockdep_free_key_range() from
> unconditionally zapping lock classes into only zapping lock classes if
> debug_lock == true. Since the new behavior can cause cat /proc/lockdep to
> crash due to a NULL pointer dereference, restore the pre-v5.1 behavior.

Can you elaborate on this NULL dereference please, and why this patch fixes
it?

> 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 | 23 ++++++-----------------
>  1 file changed, 6 insertions(+), 17 deletions(-)
> 
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 34cdcbedda49..70480e4f8f5d 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -4689,8 +4689,7 @@ static void free_zapped_rcu(struct rcu_head *ch)
>  		return;
>  
>  	raw_local_irq_save(flags);
> -	if (!graph_lock())
> -		goto out_irq;
> +	arch_spin_lock(&lockdep_lock);

This also throws out the recursion counting. Is that ok?

Will