Re: [PATCH] irqchip/msi-lib: Fix fwnode refcount in msi_lib_irq_domain_select()

[Jonathan Cameron <Jonathan.Cameron@huawei.com>]

On Tue, 5 Aug 2025 11:23:04 +0200
Lorenzo Pieralisi <lpieralisi@kernel.org> wrote:

> On Tue, Aug 05, 2025 at 10:31:32AM +0200, Thomas Gleixner wrote:
> > On Mon, Aug 04 2025 at 16:55, Lorenzo Pieralisi wrote:  
> > >
> > > msi_lib_irq_domain_select() is used in other arches, I could not
> > > test on those (don't know if they have non-[DT/irqchip/acpi] specific
> > > fwnodes) - from a fwnode interface perspective I think that this patch
> > > does the right thing, it should not add any issue to existing code
> > > to the best of my knowledge but it has to be verified.  
> > 
> > fwnode handles are architecture and firmware agnostic.  
> 
> Yep, though to make sure this does not trigger regressions I started
> checking (ie I am adding an additional fwnode_handle_get/put() in there),
> some fwnode helpers (eg fwnode_find_reference()) returns an error
> pointer rather than NULL on error, it looks like calling
> fwnode_handle_put() on that value when OF is in use is not a good idea
> (ie of_node_put() checks for NULL and dereference).
> 
> There is code out there that implicitly assumes what fwnode types
> are used behind the fwnode_* interface or I am missing something.
> 
> It is not arch dependent but it looks like it depends on what fwnodes
> arches use - that's where my caution stems from, nothing else.
> 

For the many DEFINE_FREE() uses there is a check of IS_ERR_OR_NULL()

E.g. Here it would be 
DEFINE_FREE(fwnode_handle, struct fwnode_handle *, if (!IS_ERR_OR_NULL(_T)) fwnode_handle_put(_T));

IIRC this one was an early use of DEFINE_FREE() and later discussions
argued for always adding that check purely to allow the compiler
to potentially optimize away the call.  Sounds like it would be
more generally helpful here and I can't immediately spot any negatives.

Jonathan


> Thanks,
> Lorenzo
>