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

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Message-ID: <CAJZ5v0jUsPHfo7M3rocR2AFK43LG1p21D_S=t_MhcpGd2wLy6A@mail.gmail.com>
Date:   Tue, 26 May 2020 10:52:23 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        "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 Mon, May 25, 2020 at 11:31 AM Ulf Hansson <ulf.hansson@...aro.org> wrote:
>
> 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>

Thanks!

Given the lack of other comments, I'm applying this patch as 5.8 material.