linux-kernel - Re: [PATCH] PM: runtime: clk: Fix clk_pm_runtime

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Date:   Fri, 22 May 2020 20:39:38 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Marek Szyprowski <m.szyprowski@...sung.com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Subject: Re: [PATCH] PM: runtime: clk: Fix clk_pm_runtime_get() error path

On Fri, May 22, 2020 at 7:19 AM Marek Szyprowski
<m.szyprowski@...sung.com> wrote:
>
> Hi Rafael,
>
> On 21.05.2020 19:08, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >
> > clk_pm_runtime_get() assumes that the PM-runtime usage counter will
> > be dropped by pm_runtime_get_sync() on errors, which is not the case,
> > so PM-runtime references to devices acquired by the former are leaked
> > on errors returned by the latter.
> >
> > Fix this by modifying clk_pm_runtime_get() to drop the reference if
> > pm_runtime_get_sync() returns an error.
> >
> > Fixes: 9a34b45397e5 clk: Add support for runtime PM
> > Cc: 4.15+ <stable@...r.kernel.org> # 4.15+
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>
> Frankly, I would rather fix the runtime_get_sync() instead of fixing the
> return path everywhere in the kernel. The current behavior of the
> pm_runtime_get_sync() is completely counter-intuitive then. I bet that
> in the 99% of the places where it is being called assume that no special
> fixup is needed in case of failure. This is one of the most common
> runtime PM related function and it is really a common pattern in the
> drivers to call:
>
> pm_runtime_get_sync()
>
> do something with the hardware
>
> pm_runtime_put()
>
> Do you really want to fix the error paths of the all such calls?

No, I don't, and that's why I'm proposing this patch.

The caller that does what you said above is OK now and if the behavior
of pm_runtime_get_sync() changed, that caller would need to be
updated.

OTOH, a caller that fails to drop the reference on an error returned
by pm_runtime_get_sync() is buggy (and has ever been so).

I'd rather update the buggy callers than the ones that are OK.

Thanks!

>
>
> > ---
> >   drivers/clk/clk.c |    6 +++++-
> >   1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > Index: linux-pm/drivers/clk/clk.c
> > ===================================================================
> > --- linux-pm.orig/drivers/clk/clk.c
> > +++ linux-pm/drivers/clk/clk.c
> > @@ -114,7 +114,11 @@ static int clk_pm_runtime_get(struct clk
> >               return 0;
> >
> >       ret = pm_runtime_get_sync(core->dev);
> > -     return ret < 0 ? ret : 0;
> > +     if (ret < 0) {
> > +             pm_runtime_put_noidle(core->dev);
> > +             return ret;
> > +     }
> > +     return 0;
> >   }
> >
> >   static void clk_pm_runtime_put(struct clk_core *core)
> >
> >
> >
> >
> Best regards
> --
> Marek Szyprowski, PhD
> Samsung R&D Institute Poland
>