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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Thu, 14 Jan 2021 20:38:37 +0100
Message-ID: <CAJZ5v0jREeXtYa8LAiLoRBG1ok66BR86MCQxrQ1WPR5qbhTKbw@mail.gmail.com>
Subject: Re: [PATCH] driver core: Extend device_is_dependent()
To:     Saravana Kannan <saravanak@google.com>
Cc:     "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux PM <linux-pm@vger.kernel.org>,
        Stephan Gerhold <stephan@gerhold.net>
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On Thu, Jan 14, 2021 at 8:32 PM Saravana Kannan <saravanak@google.com> wrote:
>
> On Thu, Jan 14, 2021 at 10:41 AM Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
> >
> > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> >
> > When adding a new device link, device_is_dependent() is used to
> > check whether or not the prospective supplier device does not
> > depend on the prospective consumer one to avoid adding loops
> > to the graph of device dependencies.
> >
> > However, device_is_dependent() does not take the ancestors of
> > the target device into account, so it may not detect an existing
> > reverse dependency if, for example, the parent of the target
> > device depends on the device passed as its first argument.
> >
> > For this reason, extend device_is_dependent() to also check if
> > the device passed as its first argument is an ancestor of the
> > target one and return 1 if that is the case.
> >
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > Reported-by: Stephan Gerhold <stephan@gerhold.net>
> > ---
> >  drivers/base/core.c |   12 +++++++++++-
> >  1 file changed, 11 insertions(+), 1 deletion(-)
> >
> > Index: linux-pm/drivers/base/core.c
> > ===================================================================
> > --- linux-pm.orig/drivers/base/core.c
> > +++ linux-pm/drivers/base/core.c
> > @@ -208,6 +208,16 @@ int device_links_read_lock_held(void)
> >  #endif
> >  #endif /* !CONFIG_SRCU */
> >
> > +static bool device_is_ancestor(struct device *dev, struct device *target)
> > +{
> > +       while (target->parent) {
> > +               target = target->parent;
> > +               if (dev == target)
> > +                       return true;
> > +       }
> > +       return false;
> > +}
> > +
> >  /**
> >   * device_is_dependent - Check if one device depends on another one
> >   * @dev: Device to check dependencies for.
> > @@ -221,7 +231,7 @@ int device_is_dependent(struct device *d
> >         struct device_link *link;
> >         int ret;
> >
> > -       if (dev == target)
> > +       if (dev == target || device_is_ancestor(dev, target))
> >                 return 1;
> >
> >         ret = device_for_each_child(dev, target, device_is_dependent);
> >
>
> The code works, but it's not at all obvious what it's doing. Because,
> at first glance, it's easy to mistakenly think that it's trying to
> catch this case:
> dev <- child1 <- child2 <- target
>
> Maybe it's clearer if we do this check inside the loop?

Which of the loops do you mean?  There are two of them and both need
to do the check in each step AFAICS.

> Something like:
>
>                 if (link->consumer == target ||
> device_is_ancestor(link->consumer, target))
>                         return 1;

So would this be sufficient?