linux-kernel - Re: [PATCH v2 2/2] lockdep: Fix merging of hlocks with non-zero references

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Message-ID: <20190527151438.GF2623@hirez.programming.kicks-ass.net>
Date:   Mon, 27 May 2019 17:14:38 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Imre Deak <imre.deak@...el.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ville Syrjälä 
        <ville.syrjala@...ux.intel.com>, Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH v2 2/2] lockdep: Fix merging of hlocks with non-zero
 references

On Fri, May 24, 2019 at 11:15:09PM +0300, Imre Deak wrote:
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 967352d32af1..9e2a4ab6c731 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -3637,6 +3637,11 @@ print_lock_nested_lock_not_held(struct task_struct *curr,
>  
>  static int __lock_is_held(const struct lockdep_map *lock, int read);
>  
> +static int hlock_reference(int reference)
> +{
> +	return reference ? : 1;
> +}
> +
>  /*
>   * This gets called for every mutex_lock*()/spin_lock*() operation.
>   * We maintain the dependency maps and validate the locking attempt:
> @@ -3702,17 +3707,15 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
>  	if (depth) {
>  		hlock = curr->held_locks + depth - 1;
>  		if (hlock->class_idx == class_idx && nest_lock) {
> -			if (hlock->references) {
> -				/*
> -				 * Check: unsigned int references overflow.
> -				 */
> -				if (DEBUG_LOCKS_WARN_ON(hlock->references == UINT_MAX))

What tree is this against? Afaict this is still 12 bits ?!

> -					return 0;
> +			/*
> +			 * Check: unsigned int references overflow.
> +			 */
> +			if (DEBUG_LOCKS_WARN_ON(hlock_reference(hlock->references) >
> +						UINT_MAX - hlock_reference(references)))

Idem. Also very weird overflow check..

> +				return 0;
>  
> -				hlock->references++;
> -			} else {
> -				hlock->references = 2;
> -			}
> +			hlock->references = hlock_reference(hlock->references) +
> +					    hlock_reference(references);
>  
>  			return 2;
>  		}
> -- 
> 2.17.1
>