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

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Date: Thu, 4 Jan 2024 08:37:23 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Lukas Wunner <lukas@...ner.de>
Cc: Dan Williams <dan.j.williams@...el.com>, 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>, 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 Thu, 4 Jan 2024 at 08:02, Lukas Wunner <lukas@...ner.de> wrote:
>
> 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/
>

AIUI, Peter is referring to constant propagation of compile time
constant pointers here, not pointer variables where the NULL check is
elided if the variable has already been dereferenced.