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From:   Michel Lespinasse <walken@google.com>
Date:   Thu, 30 Apr 2020 00:51:02 -0700
Message-ID: <CANN689FBczsBm=bYPfs1saUEeUq+oxLWnr8xfwtOstQkvJmwOA@mail.gmail.com>
Subject: Re: [RFC][PATCH 1/7] rbtree: Add generic add and find helpers
To:     Juri Lelli <juri.lelli@redhat.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Oleg Nesterov <oleg@redhat.com>, irogers@google.com,
        Vincent Guittot <vincent.guittot@linaro.org>
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On Thu, Apr 30, 2020 at 12:28 AM Juri Lelli <juri.lelli@redhat.com> wrote:
> > --- a/include/linux/rbtree.h
> > +++ b/include/linux/rbtree.h
> > @@ -141,12 +141,18 @@ static inline void rb_insert_color_cache
> >       rb_insert_color(node, &root->rb_root);
> >  }
> >
> > -static inline void rb_erase_cached(struct rb_node *node,
> > +static inline bool rb_erase_cached(struct rb_node *node,
> >                                  struct rb_root_cached *root)
> >  {
> > -     if (root->rb_leftmost == node)
> > +     bool leftmost = false;
> > +
> > +     if (root->rb_leftmost == node) {
> >               root->rb_leftmost = rb_next(node);
>
> Think we need
>
>  if (root->rb_leftmost)
>
> > +             leftmost = true;
>
> DEADLINE crashes w/o that.

I think Peter's code is correct; after removing the only node in an
rbtree rb_leftmost should be NULL.

The issue appears to be in dequeue_pushable_dl_task unconditionally
dereferencing the pointer returned by rb_first_cached(), which may be
NULL. I'm not sure what the correct behavior is though, i.e. what
dl_rq->earliest_dl.next should be set to if the rbtree ends up empty.
Current code (before Peter's changes) preserves the existing
dl_rq->earliest_dl.next value in that case, which seems very weird to
me (and worthy of a comment if it's correct).