linux-kernel - Re: [PATCH v5 8/9] PCI: Define scoped based management functions

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Message-ID: <6596539c9729e_8dc6829476@dwillia2-xfh.jf.intel.com.notmuch>
Date: Wed, 3 Jan 2024 22:43:40 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: Lukas Wunner <lukas@...ner.de>, Ira Weiny <ira.weiny@...el.com>
CC: Dan Williams <dan.j.williams@...el.com>, Jonathan Cameron
	<jonathan.cameron@...wei.com>, Smita Koralahalli
	<Smita.KoralahalliChannabasappa@....com>, Shiju Jose <shiju.jose@...wei.com>,
	Yazen Ghannam <yazen.ghannam@....com>, Davidlohr Bueso <dave@...olabs.net>,
	Dave Jiang <dave.jiang@...el.com>, Alison Schofield
	<alison.schofield@...el.com>, Vishal Verma <vishal.l.verma@...el.com>, "Ard
 Biesheuvel" <ardb@...nel.org>, <linux-efi@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <linux-cxl@...r.kernel.org>, Bjorn Helgaas
	<bhelgaas@...gle.com>
Subject: Re: [PATCH v5 8/9] PCI: Define scoped based management functions

Lukas Wunner wrote:
> On Wed, Dec 20, 2023 at 04:17:35PM -0800, Ira Weiny wrote:
> > --- a/include/linux/pci.h
> > +++ b/include/linux/pci.h
> > @@ -1170,6 +1170,7 @@ int pci_get_interrupt_pin(struct pci_dev *dev, struct pci_dev **bridge);
> >  u8 pci_common_swizzle(struct pci_dev *dev, u8 *pinp);
> >  struct pci_dev *pci_dev_get(struct pci_dev *dev);
> >  void pci_dev_put(struct pci_dev *dev);
> > +DEFINE_FREE(pci_dev_put, struct pci_dev *, if (_T) pci_dev_put(_T))
> 
> pci_dev_put() already performs a NULL pointer check internally.
> Why duplicate it here?

Greg asked the same for the introduction of __free(kvfree), and Peter
clarified:

http://lore.kernel.org/r/20230814161731.GN776869@hirez.programming.kicks-ass.net

Essentially, that check is more for build-time than runtime because when
the macro is expanded the compiler can notice scenarios where @pdev is
set to NULL (likely by no_free_ptr()) and skip the call to pci_dev_put()
altogether. pci_dev_put() also happens to be out-of-line, so saving a
call when @pdev is NULL a small win in that respect as well.