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

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Date:   Fri, 11 Jan 2019 07:55:03 -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 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;
+               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.