linux-kernel - Re: [PATCH] locking/lockdep: Avoid recursion in lockdep_count_{for,back}ward

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Date:   Mon, 16 Mar 2020 11:01:10 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org,
        rcu@...r.kernel.org, "Paul E. McKenney" <paulmck@...nel.org>,
        Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>,
        Qian Cai <cai@....pw>, Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH] locking/lockdep: Avoid recursion in
 lockdep_count_{for,back}ward_deps()

On Mon, Mar 16, 2020 at 02:55:07PM +0100, Peter Zijlstra wrote:
> On Sat, Mar 14, 2020 at 09:04:22PM -0400, Joel Fernandes wrote:
> > On Fri, Mar 13, 2020 at 10:33:25AM +0100, Peter Zijlstra wrote:
> 
> > Thanks Peter and Boqun, the below patch makes sense to me. Just had some nits
> > below, otherwise:
> > > @@ -1719,11 +1725,11 @@ unsigned long lockdep_count_forward_deps
> > >  	this.class = class;
> > >  
> > >  	raw_local_irq_save(flags);
> > > -	current->lockdep_recursion = 1;
> > > +	current->lockdep_recursion++;
> > >  	arch_spin_lock(&lockdep_lock);
> > >  	ret = __lockdep_count_forward_deps(&this);
> > >  	arch_spin_unlock(&lockdep_lock);
> > > -	current->lockdep_recursion = 0;
> > > +	current->lockdep_recursion--;
> > 
> > This doesn't look like it should recurse. Why not just use the
> > lockdep_recursion_finish() helper here?
> 
> I chose to only add that to the sites that check recursion on entry.

Makes sense, thank you Peter.

 - Joel