On Thu, 31 Oct 2024, Yazen Ghannam wrote:
> 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@xxxxxxx>
> > > > [...]
> > > > +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);
Yes, that is the recommendation. It says "always" so not only the cases
where guard() or other __free()s are used.
Of course this only applies to use of __free(), other variables should
still be declared in the usual place and not spread around.
--
i.
