linux-kernel - Re: [PATCH v2] driver core: Extend device_is

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Date:   Tue, 19 Jan 2021 14:23:44 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Saravana Kannan <saravanak@...gle.com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Stephan Gerhold <stephan@...hold.net>
Subject: Re: [PATCH v2] driver core: Extend device_is_dependent()

On Mon, Jan 18, 2021 at 11:03 PM Saravana Kannan <saravanak@...gle.com> wrote:
>
> On Fri, Jan 15, 2021 at 10:30 AM Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> >
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >
> > If the device passed as the target (second argument) to
> > device_is_dependent() is not completely registered (that is, it has
> > been initialized, but not added yet), but the parent pointer of it
> > is set, it may be missing from the list of the parent's children
> > and device_for_each_child() called by device_is_dependent() cannot
> > be relied on to catch that dependency.
> >
> > For this reason, modify device_is_dependent() to check the ancestors
> > of the target device by following its parent pointer in addition to
> > the device_for_each_child() walk.
> >
> > Fixes: 9ed9895370ae ("driver core: Functional dependencies tracking support")
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > Reported-by: Stephan Gerhold <stephan@...hold.net>
> > Tested-by: Stephan Gerhold <stephan@...hold.net>
> > ---
> >
> > -> v2:
> >    * Improve the changelog.
> >    * Add a comment to explain the reason for the extra check.
> >    * Add tags.
> >
> >    No code changes.
> >
> > ---
> >  drivers/base/core.c |   17 ++++++++++++++++-
> >  1 file changed, 16 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 +231,12 @@ int device_is_dependent(struct device *d
> >         struct device_link *link;
> >         int ret;
> >
> > -       if (dev == target)
> > +       /*
> > +        * The "ancestors" check is needed to catch the case when the target
> > +        * device has not been completely initialized yet and it is still
> > +        * missing from the list of children of its parent device.
> > +        */
> > +       if (dev == target || device_is_ancestor(dev, target))
> >                 return 1;
> >
> >         ret = device_for_each_child(dev, target, device_is_dependent);
> >
>
> Reviewed-by: Saravana Kannan <saravanak@...gle.com>
>
> fw_devlink_relax_cycle() needs a similar fix. Want me to handle that
> as a separate patch to driver-core-next? Or do you want to combine
> that in this patch?

I would prefer that to be a different patch because of the Fixes: tag.

Also fw_devlink_relax_cycle() is in linux-next only for now, so it is
better to avoid mixing a mainline fix with updating that one.