On Thu, Oct 31, 2024 at 05:42:34PM +0200, Ilpo Järvinen wrote:
> 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.
>
Ah right. Will make the change.
Thanks,
Yazen
