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

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Date:   Mon, 27 May 2019 19:21:49 +0300
From:   Imre Deak <imre.deak@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 Mon, May 27, 2019 at 06:06:18PM +0200, Peter Zijlstra wrote:
> On Mon, May 27, 2019 at 05:14:38PM +0200, Peter Zijlstra wrote:
> > 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;
> > >  		}
> 
> I think you wanted something like this: ?
> 
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -3731,6 +3731,11 @@ print_lock_nested_lock_not_held(struct t
>  
>  static int __lock_is_held(const struct lockdep_map *lock, int read);
>  
> +static inline int hlock_references(struct held_lock *hlock)
> +{
> +	return hlock->references ? : 1;
> +}
> +
>  /*
>   * This gets called for every mutex_lock*()/spin_lock*() operation.
>   * We maintain the dependency maps and validate the locking attempt:
> @@ -3796,17 +3801,14 @@ static int __lock_acquire(struct lockdep
>  	if (depth) {
>  		hlock = curr->held_locks + depth - 1;
>  		if (hlock->class_idx == class_idx && nest_lock) {
> -			if (hlock->references) {
> -				/*
> -				 * Check: unsigned int references:12, overflow.
> -				 */
> -				if (DEBUG_LOCKS_WARN_ON(hlock->references == (1 << 12)-1))
> -					return 0;
> -
> -				hlock->references++;
> -			} else {
> -				hlock->references = 2;
> -			}
> +			if (!references)
> +				references++;
> +
> +			hlock->references = hlock_references(hlock) + references;
> +
> +			/* Overflow */
> +			if (DEBUG_LOCKS_WARN_ON(hlock->references < references))
> +				return 0;

Yes, it works.

>  
>  			return 2;
>  		}