linux-kernel - Re: [PATCH 06/16] x86/amd_nb: Simplify root device search

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Message-ID: <20241031153444.GB1511886@yaz-khff2.amd.com>
Date: Thu, 31 Oct 2024 11:34:44 -0400
From: Yazen Ghannam <yazen.ghannam@....com>
To: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc: "Zhuo, Qiuxu" <qiuxu.zhuo@...el.com>,
	"linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Luck, Tony" <tony.luck@...el.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"avadhut.naik@....com" <avadhut.naik@....com>,
	"john.allen@....com" <john.allen@....com>,
	"mario.limonciello@....com" <mario.limonciello@....com>,
	"bhelgaas@...gle.com" <bhelgaas@...gle.com>,
	"Shyam-sundar.S-k@....com" <Shyam-sundar.S-k@....com>,
	"richard.gong@....com" <richard.gong@....com>,
	"jdelvare@...e.com" <jdelvare@...e.com>,
	"linux@...ck-us.net" <linux@...ck-us.net>,
	"clemens@...isch.de" <clemens@...isch.de>,
	"hdegoede@...hat.com" <hdegoede@...hat.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-hwmon@...r.kernel.org" <linux-hwmon@...r.kernel.org>,
	"platform-driver-x86@...r.kernel.org" <platform-driver-x86@...r.kernel.org>,
	"naveenkrishna.chatradhi@....com" <naveenkrishna.chatradhi@....com>,
	"carlos.bilbao.osdev@...il.com" <carlos.bilbao.osdev@...il.com>
Subject: Re: [PATCH 06/16] x86/amd_nb: Simplify root device search

On Thu, Oct 31, 2024 at 12:08:20PM +0200, Ilpo Järvinen wrote:
> On Thu, 31 Oct 2024, Zhuo, Qiuxu wrote:
> 
> > > From: Yazen Ghannam <yazen.ghannam@....com>
> > > [...]
> > > +struct pci_dev *amd_node_get_root(u16 node) {
> > > +	struct pci_dev *df_f0 __free(pci_dev_put) = NULL;
> > 
> > NULL pointer initialization is not necessary.
> 
> It is, because __free() is used...
> 
> > > +	struct pci_dev *root;
> > > +	u16 cntl_off;
> > > +	u8 bus;
> > > +
> > > +	if (!boot_cpu_has(X86_FEATURE_ZEN))
> > > +		return NULL;
> 
> ...This would try to free() whatever garbage df_f0 holds...
> 
> > > +	/*
> > > +	 * D18F0xXXX [Config Address Control] (DF::CfgAddressCntl)
> > > +	 * Bits [7:0] (SecBusNum) holds the bus number of the root device for
> > > +	 * this Data Fabric instance. The segment, device, and function will be
> > > 0.
> > > +	 */
> > > +	df_f0 = amd_node_get_func(node, 0);
> 
> ...However, the recommended practice when using __free() is this (as 
> documented in include/linux/cleanup.h):
> 
>  * Given that the "__free(...) = NULL" pattern for variables defined at
>  * the top of the function poses this potential interdependency problem
>  * the recommendation is to always define and assign variables in one
>  * statement and not group variable definitions at the top of the
>  * function when __free() is used.
> 
> I know the outcome will look undesirable to some, me included, but 
> there's little that can be done to that because there's no other way for 
> the compiler to infer the order.
> 
> That being said, strictly speaking it isn't causing issue in this function 
> as is but it's still a bad pattern to initialize to = NULL because in 
> other instances it will cause problems. So better to steer away from the
> pattern entirely rather than depend on reviewers noticing the a cleaup 
> ordering problem gets introduced by some later change to the function.
>

I originally read that in the context of using a guard(). But really we
should do like this in any case, correct?

struct pci_dev *df_f0 __free(pci_dev_put) = amd_node_get_func(node, 0);

Thanks,
Yazen