linux-kernel - Re: [PATCH v2 1/6] PCI/ASPM: Add locked helper for enabling link state

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Message-ID: <20231208173932.GA798089@bhelgaas>
Date:   Fri, 8 Dec 2023 11:39:32 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Johan Hovold <johan@...nel.org>
Cc:     Johan Hovold <johan+linaro@...nel.org>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Manivannan Sadhasivam <mani@...nel.org>,
        Rob Herring <robh@...nel.org>,
        Nirmal Patel <nirmal.patel@...ux.intel.com>,
        Jonathan Derrick <jonathan.derrick@...ux.dev>,
        linux-arm-msm@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Michael Bottini <michael.a.bottini@...ux.intel.com>,
        "David E . Box" <david.e.box@...ux.intel.com>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        Kai-Heng Feng <kai.heng.feng@...onical.com>
Subject: Re: [PATCH v2 1/6] PCI/ASPM: Add locked helper for enabling link
 state

On Fri, Dec 08, 2023 at 09:00:56AM +0100, Johan Hovold wrote:
> On Thu, Dec 07, 2023 at 02:47:16PM -0600, Bjorn Helgaas wrote:
> > On Tue, Nov 28, 2023 at 09:15:07AM +0100, Johan Hovold wrote:
> > > Add a helper for enabling link states that can be used in contexts where
> > > a pci_bus_sem read lock is already held (e.g. from pci_walk_bus()).
> > > 
> > > This helper will be used to fix a couple of potential deadlocks where
> > > the current helper is called with the lock already held, hence the CC
> > > stable tag.
> 
> > As far as I can see, we end up with pci_enable_link_state() defined
> > but never called and pci_enable_link_state_locked() being called only
> > by pcie-qcom.c and vmd.c.
> 
> Correct, I mentioned this in the cover letter.

Ah, right.  I really don't like these exported locked/unlocked
interfaces because pci_bus_sem is internal to the PCI core, and the
caller shouldn't need to know or be able to specify whether it is held
or not.  They exist for now, but I think we should try to get rid of
them.

> > Can we just rename pci_enable_link_state() to
> > pci_enable_link_state_locked() and assert that pci_bus_sem is held, so
> > we don't end up with a function that's never used?
> 
> That would work too. I went with adding a new helper to facilitate
> stable backports and to mirror pci_disable_link_state(). The variants
> are simple wrappers around the implementation so there's no real cost to
> having the unused one.

Makes good sense.  There's no real machine cost to the unused one; I'm
more concerned about the human cost here.

> But it seems like you think there will never be a need to call this
> helper outside of pci_walk_bus() and if so we can drop the unlocked
> variant right away.
> 
> Would you prefer basically squashing the first three patches and mark
> the result for stable even though that patch will fail to apply to older
> kernels as the Qualcomm bits went into -rc1?
> 
> Or should I send a follow-on patch removing the unused helper after
> merging this series?

I think you did the right thing.

Bjorn