linux-kernel - Re: [PATCH] powerpc/pci: Remove useless null check before call of_node

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Date:   Sun, 24 Apr 2022 00:32:57 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Tyrel Datwyler <tyreld@...ux.ibm.com>,
        Haowen Bai <baihaowen@...zu.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc/pci: Remove useless null check before call
 of_node_put()

Tyrel Datwyler <tyreld@...ux.ibm.com> writes:
> On 4/20/22 19:52, Haowen Bai wrote:
>> No need to add null check before call of_node_put(), since the
>> implementation of of_node_put() has done it.
>> 
>> Signed-off-by: Haowen Bai <baihaowen@...zu.com>
>> ---
>>  arch/powerpc/kernel/pci_dn.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/arch/powerpc/kernel/pci_dn.c b/arch/powerpc/kernel/pci_dn.c
>> index 61571ae23953..ba3bbc9bec2d 100644
>> --- a/arch/powerpc/kernel/pci_dn.c
>> +++ b/arch/powerpc/kernel/pci_dn.c
>> @@ -357,8 +357,8 @@ void pci_remove_device_node_info(struct device_node *dn)
>> 
>>  	/* Drop the parent pci_dn's ref to our backing dt node */
>>  	parent = of_get_parent(dn);
>> -	if (parent)
>> -		of_node_put(parent);
>> +
>> +	of_node_put(parent);
>
> This whole block of code looks useless, or suspect. Examining the rest of the
> code for this function this is the only place that parent is referenced. The
> of_get_parent() call returns the parent with its refcount incremented, and then
> we turn around and call of_node_put() which drops that reference we just took.
> The comment doesn't do what it says it does. If we really need to drop a
> previous reference to the parent device node this code block would need to call
> of_node_put() twice on parent to accomplish that.

Yeah good analysis.

It used to use pdn->parent, which didn't grab  an extra reference, see
commit 14db3d52d3a2 ("powerpc/eeh: Reduce use of pci_dn::node").

The old code was:

        if (pdn->parent)
                of_node_put(pdn->parent->node);

> A closer examination is required to determine if what the comment says we need
> to do is required. If it is then the code as it exists today is leaking that
> reference AFAICS.

Yeah. This function is only called from pnv_php.c, ie. powernv PCI
hotplug, which I think gets less testing than pseries hotplug. So
possibly we are leaking references and haven't noticed, or maybe the
comment is out of date.

cheers