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

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Date:   Mon, 25 May 2020 11:30:48 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>,
        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>,
        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, 22 May 2020 at 20:39, Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> 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.

I agree.

In hindsight we should have dropped the usage count in
pm_runtime_get_sync(), when it fails. However, that's too late,
especially since there are many cases having no error handling at all
- and in those cases, that would mean the subsequent call to
pm_runtime_put() can mess up the usage count (if pm_runtime_get_sync()
failed and has already dropped the count).

So, feel free to add:

Reviewed-by: Ulf Hansson <ulf.hansson@...aro.org>

[...]

Kind regards
Uffe