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

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Date: Thu, 4 Jan 2024 08:02:35 +0100
From: Lukas Wunner <lukas@...ner.de>
To: Dan Williams <dan.j.williams@...el.com>
Cc: Ira Weiny <ira.weiny@...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

On Wed, Jan 03, 2024 at 10:43:40PM -0800, Dan Williams wrote:
> 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.

Doubtful whether that's correct.  The kernel is compiled with
-fno-delete-null-pointer-checks since commit a3ca86aea507
("Add '-fno-delete-null-pointer-checks' to gcc CFLAGS").

So these NULL pointer checks are generally not optimized away.

I've just responded to the discussion you've linked above:
https://lore.kernel.org/all/20240104065744.GA6055@wunner.de/

Thanks,

Lukas