Re: [PATCH v3 1/2] PCI: PCIe: ASPM: Introduce pcie_aspm_enabled()

[Bjorn Helgaas]

On Tue, Oct 08, 2019 at 11:27:51AM +0200, Rafael J. Wysocki wrote:
> On Tue, Oct 8, 2019 at 12:34 AM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
> > On Thu, Aug 08, 2019 at 11:55:07PM +0200, Rafael J. Wysocki wrote:
> > > From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> > >
> > > 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@xxxxxxxxx>
> > > ---
> > >
> > > 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?

Hmm, yeah, I guess that's the question.  It's not a race with another
thread changing aspm_enabled; the potential race is with another
thread removing the last child of "bridge", which will free the
link_state and set bridge->link_state = NULL.

I think it should be safe to call device-related PCI interfaces if
you're holding a reference to the device, e.g., from a driver bound to
the device or a sysfs accessor.  Since we call pcie_aspm_enabled(dev)
from a driver bound to "dev", another thread should not be able to
remove "dev" while we're using it.

I know that's a little hand-wavey, but if it weren't true, I think
we'd have a lot more locking sprinkled everywhere in the PCI core than
we do.

This has implications for Heiner's ASPM sysfs patches because we're
currently doing this in sysfs accessors:

  static ssize_t aspm_attr_show_common(struct device *dev, ...)
  {
    ...
    link = pcie_aspm_get_link(pdev);

    mutex_lock(&aspm_lock);
    enabled = link->aspm_enabled & state;
    mutex_unlock(&aspm_lock);
    ...
  }

I assume sysfs must be holding a reference that guarantees "dev" is
valid througout this code, and therefore we should not need to hold
aspm_lock.

Bjorn