linux-kernel - Re: [PATCH v3 1/2] PCI: PCIe: ASPM: Introduce pcie_aspm

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Message-ID: <CAJZ5v0iL4Rv=AQ-Qnma=wWaqBBbOdfRqu9uS9Gesa-G75oQpzQ@mail.gmail.com>
Date:   Tue, 8 Oct 2019 11:27:51 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        linux-nvme <linux-nvme@...ts.infradead.org>,
        Keith Busch <kbusch@...nel.org>,
        Mario Limonciello <Mario.Limonciello@...l.com>,
        Kai-Heng Feng <kai.heng.feng@...onical.com>,
        Keith Busch <keith.busch@...el.com>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Rajat Jain <rajatja@...gle.com>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Heiner Kallweit <hkallweit1@...il.com>
Subject: Re: [PATCH v3 1/2] PCI: PCIe: ASPM: Introduce pcie_aspm_enabled()

On Tue, Oct 8, 2019 at 12:34 AM Bjorn Helgaas <helgaas@...nel.org> wrote:
>
> [+cc Heiner]
>
> On Thu, Aug 08, 2019 at 11:55:07PM +0200, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >
> > Add a function checking whether or not PCIe ASPM has been enabled for
> > a given device.
> >
> > It will be used by the NVMe driver to decide how to handle the
> > device during system suspend.
> >
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > ---
> >
> > v2 -> v3:
> >   * Make the new function return bool.
> >   * Change its name back to pcie_aspm_enabled().
> >   * Fix kerneldoc comment formatting.
> >
> > -> v2:
> >   * Move the PCI/PCIe ASPM changes to a separate patch.
> >   * Add the _mask suffix to the new function name.
> >   * Add EXPORT_SYMBOL_GPL() to the new function.
> >   * Avoid adding an unnecessary blank line.
> >
> > ---
> >  drivers/pci/pcie/aspm.c |   20 ++++++++++++++++++++
> >  include/linux/pci.h     |    3 +++
> >  2 files changed, 23 insertions(+)
> >
> > Index: linux-pm/drivers/pci/pcie/aspm.c
> > ===================================================================
> > --- linux-pm.orig/drivers/pci/pcie/aspm.c
> > +++ linux-pm/drivers/pci/pcie/aspm.c
> > @@ -1170,6 +1170,26 @@ static int pcie_aspm_get_policy(char *bu
> >  module_param_call(policy, pcie_aspm_set_policy, pcie_aspm_get_policy,
> >       NULL, 0644);
> >
> > +/**
> > + * pcie_aspm_enabled - Check if PCIe ASPM has been enabled for a device.
> > + * @pci_device: Target device.
> > + */
> > +bool pcie_aspm_enabled(struct pci_dev *pci_device)
> > +{
> > +     struct pci_dev *bridge = pci_upstream_bridge(pci_device);
> > +     bool ret;
> > +
> > +     if (!bridge)
> > +             return false;
> > +
> > +     mutex_lock(&aspm_lock);
> > +     ret = bridge->link_state ? !!bridge->link_state->aspm_enabled : false;
> > +     mutex_unlock(&aspm_lock);
>
> Why do we need to acquire aspm_lock here?  We aren't modifying
> anything, and I don't think we're preventing a race.  If this races
> with another thread that changes aspm_enabled, we'll return either the
> old state or the new one, and I think that's still the case even if we
> don't acquire aspm_lock.

Well, if we can guarantee that pci_remove_bus_device() will never be
called in parallel with this helper, then I agree, but can we
guarantee that?