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

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Date:   Tue, 19 Jan 2021 16:09:14 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Stephan Gerhold <stephan@...hold.net>,
        Saravana Kannan <saravanak@...gle.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: Re: [PATCH v2] driver core: Extend device_is_dependent()

On Fri, Jan 15, 2021 at 7:31 PM 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>

Greg, are you going to pick up this one or do you want me to take care of it?

It has been reviewed by Saravana.

> ---
>
> -> 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);
>
>
>