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

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Date:   Fri, 11 Jan 2019 17:55:29 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Bart Van Assche <bvanassche@....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, 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);
>         }
> 
> Please let me know if you need more information.
> 
> Bart.
>