linux-kernel - Re: [PATCH] PCI: kirin: Fix of_node

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Date:   Wed, 3 Nov 2021 09:30:59 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Wan Jiabing <wanjiabing@...o.com>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Rob Herring <robh@...nel.org>
Cc:     Xiaowei Song <songxiaowei@...ilicon.com>,
        Binghui Wang <wangbinghui@...ilicon.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, jiabing.wan@...com
Subject: Re: [PATCH] PCI: kirin: Fix of_node_put() issue in pcie-kirin

[+to Mauro, author of code being changed,
Rob for "of_pci_get_devfn()" naming question]

On Wed, Nov 03, 2021 at 02:25:18AM -0400, Wan Jiabing wrote:
> Fix following coccicheck warning:
> ./drivers/pci/controller/dwc/pcie-kirin.c:414:2-34: WARNING: Function
> for_each_available_child_of_node should have of_node_put() before return.
> 
> Early exits from for_each_available_child_of_node should decrement the
> node reference counter. Replace return by goto here.
> 
> Signed-off-by: Wan Jiabing <wanjiabing@...o.com>
> ---
>  drivers/pci/controller/dwc/pcie-kirin.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/controller/dwc/pcie-kirin.c b/drivers/pci/controller/dwc/pcie-kirin.c
> index 06017e826832..23a2c076ce53 100644
> --- a/drivers/pci/controller/dwc/pcie-kirin.c
> +++ b/drivers/pci/controller/dwc/pcie-kirin.c
> @@ -422,7 +422,8 @@ static int kirin_pcie_parse_port(struct kirin_pcie *pcie,
>  			pcie->num_slots++;
>  			if (pcie->num_slots > MAX_PCI_SLOTS) {
>  				dev_err(dev, "Too many PCI slots!\n");
> -				return -EINVAL;
> +				ret = -EINVAL;
> +				goto put_node;
>  			}
>  
>  			ret = of_pci_get_devfn(child);

This is a change to the code added here:
  https://git.kernel.org/pub/scm/linux/kernel/git/helgaas/pci.git/commit/?id=31bd24f0cfe0

This fix looks right to me; all the other early exits from the inner
loop drop the "child" reference.

But this is a nested loop and the *outer* loop also increments
refcounts, and I don't see that outer loop reference on "parent" being
dropped at all:

    for_each_available_child_of_node(node, parent) { 
      for_each_available_child_of_node(parent, child) {
	...
	if (error)
	  goto put_node;
      }
    }

  put_node:
    of_node_put(child);

The "of_pci_get_devfn()" immediately after is unrelated, but possibly
a confusing name.  "Get" often suggests a reference count being
increased, but that's not the case with of_pci_get_devfn().

I want to fix this before sending a pull request to Linus, and I can
easily squash it into a local branch, but I need an ack from Mauro
that this patch is correct and also a fix or explanation for the outer
loop reference situation.

Bjorn