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

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Message-ID: <ZXLNONZRafTkOk9U@hovoldconsulting.com>
Date:   Fri, 8 Dec 2023 09:00:56 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Bjorn Helgaas <helgaas@...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 Thu, Dec 07, 2023 at 02:47:16PM -0600, Bjorn Helgaas wrote:
> [+cc Kai-Heng]
> 
> 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.

> 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.

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?

The end-result will be identical.

Johan