linux-kernel - Re: [RFC v2] driver core: Fix repeated device_is

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Date:   Thu, 7 Jul 2022 13:41:57 +0300
From:   Abel Vesa <abel.vesa@...aro.org>
To:     Saravana Kannan <saravanak@...gle.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        linux-arm-msm@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC v2] driver core: Fix repeated device_is_dependent check for
 same link

On 22-07-06 21:51:34, Saravana Kannan wrote:
> On Wed, Jul 6, 2022 at 8:54 AM Abel Vesa <abel.vesa@...aro.org> wrote:
> >
> > In case of a cyclic dependency, if the supplier is not yet available,
> > the parent of the supplier is checked for dependency. But if there are
> > more than one suppliers with the same parent, the first check returns
> > true while the next ones skip that specific link entirely because of
> > having DL_FLAG_MANAGED and DL_FLAG_SYNC_STATE_ONLY set, which is what
> > the relaxing of the link does. But if we check for the target being
> > a consumer before the check for those flags, we can check as many
> > times as needed the same link and it will always return true, This is
> > safe to do, since the relaxing of the link will be done only once
> > because those flags will be set and it will bail early.
> >
> > Signed-off-by: Abel Vesa <abel.vesa@...aro.org>
> > ---
> >  drivers/base/core.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/base/core.c b/drivers/base/core.c
> > index 753e7cca0f40..2c3b860dfe80 100644
> > --- a/drivers/base/core.c
> > +++ b/drivers/base/core.c
> > @@ -297,13 +297,13 @@ int device_is_dependent(struct device *dev, void *target)
> >                 return ret;
> >
> >         list_for_each_entry(link, &dev->links.consumers, s_node) {
> > +               if (link->consumer == target)
> > +                       return 1;
> > +
> >                 if ((link->flags & ~DL_FLAG_INFERRED) ==
> >                     (DL_FLAG_SYNC_STATE_ONLY | DL_FLAG_MANAGED))
> >                         continue;
>
> Thanks for trying to fix this issue, but I'll have to Nack this patch.
>
> The whole point of the SYNC_STATE_ONLY flag is to allow cycles. It's
> needed to maintain correctness of sync_state(). I think I described
> those in the commit text that added the SYNC_STATE_ONLY flag. Check it
> out if you are interested. So this change of yours will break
> sync_state() functionality.
>

Fair enough.

> There's a bunch of nuance to fixing the dual cycle issue and I don't
> mind fixing this myself in a week or two if you can wait.
>

Please cc me on it.

> Thanks,
> Saravana
>
> >
> > -               if (link->consumer == target)
> > -                       return 1;
> > -
> >                 ret = device_is_dependent(link->consumer, target);
> >                 if (ret)
> >                         break;
> > --
> > 2.34.3
> >