linux-kernel - Re: [PATCH 1/1] PCI/ASPM: Remove pcie_aspm

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Date:   Thu, 10 Oct 2019 18:28:29 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Linux PCI <linux-pci@...r.kernel.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Sagi Grimberg <sagi@...mberg.me>,
        Mario Limonciello <Mario.Limonciello@...l.com>,
        Keith Busch <keith.busch@...el.com>,
        Kai-Heng Feng <kai.heng.feng@...onical.com>,
        Rajat Jain <rajatja@...gle.com>,
        Christoph Hellwig <hch@....de>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-nvme <linux-nvme@...ts.infradead.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH 1/1] PCI/ASPM: Remove pcie_aspm_enabled() unnecessary locking

On Thu, Oct 10, 2019 at 2:48 PM Bjorn Helgaas <helgaas@...nel.org> wrote:
>
> From: Bjorn Helgaas <bhelgaas@...gle.com>
>
> The lifetime of the link_state structure (bridge->link_state) is not the
> same as the lifetime of "bridge" itself.  The link_state is allocated by
> pcie_aspm_init_link_state() after children of the bridge have been
> enumerated, and it is deallocated by pcie_aspm_exit_link_state() after all
> children of the bridge (but not the bridge itself) have been removed.
>
> Previously pcie_aspm_enabled() acquired aspm_lock to ensure that
> link_state was not deallocated while we're looking at it.  But the fact
> that the caller of pcie_aspm_enabled() holds a reference to @pdev means
> there's always at least one child of the bridge, which means link_state
> can't be deallocated.
>
> Remove the unnecessary locking in pcie_aspm_enabled().
>
> Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.com>

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

(also for the further changes you and Christoph have been discussing).

> ---
>  drivers/pci/pcie/aspm.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
> index 652ef23bba35..f5c7138a34aa 100644
> --- a/drivers/pci/pcie/aspm.c
> +++ b/drivers/pci/pcie/aspm.c
> @@ -1172,20 +1172,20 @@ module_param_call(policy, pcie_aspm_set_policy, pcie_aspm_get_policy,
>  /**
>   * pcie_aspm_enabled - Check if PCIe ASPM has been enabled for a device.
>   * @pdev: Target device.
> + *
> + * Relies on the upstream bridge's link_state being valid.  The link_state
> + * is deallocated only when the last child of the bridge (i.e., @pdev or a
> + * sibling) is removed, and the caller should be holding a reference to
> + * @pdev, so this should be safe.
>   */
>  bool pcie_aspm_enabled(struct pci_dev *pdev)
>  {
>         struct pci_dev *bridge = pci_upstream_bridge(pdev);
> -       bool ret;
>
>         if (!bridge)
>                 return false;
>
> -       mutex_lock(&aspm_lock);
> -       ret = bridge->link_state ? !!bridge->link_state->aspm_enabled : false;
> -       mutex_unlock(&aspm_lock);
> -
> -       return ret;
> +       return bridge->link_state ? !!bridge->link_state->aspm_enabled : false;
>  }
>  EXPORT_SYMBOL_GPL(pcie_aspm_enabled);
>
> --
> 2.23.0.581.g78d2f28ef7-goog
>