linux-kernel - Re: [PATCH v6 00/16] locking/lockdep: Add support for dynamic keys

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Date:   Fri, 11 Jan 2019 09:01:41 -0800
From:   Bart Van Assche <bvanassche@....org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, tj@...nel.org, longman@...hat.com,
        johannes.berg@...el.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 00/16] locking/lockdep: Add support for dynamic keys

On Fri, 2019-01-11 at 17:55 +0100, Peter Zijlstra wrote:
> On Fri, Jan 11, 2019 at 07:55:03AM -0800, Bart Van Assche wrote:
> > On Fri, 2019-01-11 at 13:48 +0100, Peter Zijlstra wrote:
> > > I spotted this new v6 in my inbox and have rebased to it.
> > 
> > Thanks!
> > 
> > > On Wed, Jan 09, 2019 at 01:01:48PM -0800, Bart Van Assche wrote:
> > > 
> > > > The changes compared to v5 are:
> > > > - Modified zap_class() such that it doesn't try to free a list entry that
> > > >   is already being freed.
> > > 
> > > I however have a question on this; this seems wrong. Once a list entry
> > > is enqueued it should not be reachable anymore. If we can reach an entry
> > > after call_rcu() happened, we've got a problem.
> > 
> > Apparently I confused you - sorry that I was not more clear. What I meant is
> > that I changed a single if test into a loop. The graph lock is held while that
> > loop is being executed so the code below is serialized against the code called
> > from inside the RCU callback:
> > 
> > @@ -4574,8 +4563,9 @@ static void zap_class(struct pending_free *pf, struct lock
> > _class *class)
> >                 entry = list_entries + i;
> >                 if (entry->class != class && entry->links_to != class)
> >                         continue;
> > -               if (__test_and_set_bit(i, pf->list_entries_being_freed))
> > +               if (list_entry_being_freed(i))
> >                         continue;
> 
> Yes, it is the above change that caught my eye.. That checks _both_ your
> lists. One is your current open one (@pf), but the other could already
> be pending the call_rcu().
> 
> So my question is why do we have to check both ?! How come the old code,
> that only checked @pf, is wrong?
> 
> > +               set_bit(i, pf->list_entries_being_freed);
> >                 nr_list_entries--;
> >                 list_del_rcu(&entry->entry);
> >         }

The list_del_rcu() call must only happen once. I ran into complaints reporting that
the list_del_rcu() call triggered list corruption. This change made these complaints
disappear.

Bart.