linux-kernel - Re: [PATCH v2] MIPS: Fix incorrect NULL check in local_flush_tlb

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Date:	Sun, 6 Jul 2014 12:16:38 +0100 (BST)
From:	"Maciej W. Rozycki" <macro@...ux-mips.org>
To:	Emil Goode <emilgoode@...il.com>
cc:	Ralf Baechle <ralf@...ux-mips.org>,
	Jonas Gorski <jogo@...nwrt.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	John Crispin <blogic@...nwrt.org>,
	MIPS Mailing List <linux-mips@...ux-mips.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kernel-janitors@...r.kernel.org
Subject: Re: [PATCH v2] MIPS: Fix incorrect NULL check in
 local_flush_tlb_page()

On Sun, 6 Jul 2014, Emil Goode wrote:

> > > diff --git a/arch/mips/mm/tlb-r3k.c b/arch/mips/mm/tlb-r3k.c
> > > index d657493..6546758 100644
> > > --- a/arch/mips/mm/tlb-r3k.c
> > > +++ b/arch/mips/mm/tlb-r3k.c
> > > @@ -158,7 +158,7 @@ void local_flush_tlb_page(struct vm_area_struct *vma, unsigned long page)
> > >  {
> > >         int cpu = smp_processor_id();
> > >
> > > -       if (!vma || cpu_context(cpu, vma->vm_mm) != 0) {
> > > +       if (vma && cpu_context(cpu, vma->vm_mm) != 0) {
> > 
> > Sorry for replying "too late", but grepping through the kernel code I
> > fail to find any caller that does not dereference vma before calling
> > (local)flush_tlb_page(). Also both tlb-4k and tlb-8k assume vma cannot
> > be NULL, so I would say it is safe to assume vma is never NULL, and
> > the NULL check can be removed completely.
> > 
> > Also it looks like this "bug" was there since at least 2.6.12, and
> > never seem to have bitten anyone.
> 
> Yes, the bug pre-dates GIT history and I agree that it is most unlikely
> that it ever caused a problem. I will send a new patch that removes the
> NULL check of vma.

 Well, as at 2.4.26 (picked randomly) code in arch/mips/mm/tlb-r4k.c had 
the same check:

void local_flush_tlb_page(struct vm_area_struct *vma, unsigned long page)
{
	int cpu = smp_processor_id();

	if (!vma || cpu_context(cpu, vma->vm_mm) != 0) {
		unsigned long flags;
		[...]

but code in arch/mips64/mm/tlb-r4k.c did not:

void local_flush_tlb_page(struct vm_area_struct *vma, unsigned long page)
{
	int cpu = smp_processor_id();

	if (cpu_context(cpu, vma->vm_mm) != 0) {
		unsigned long flags;
		[...]

and the current situation is an artefact from the 32-bit and 64-bit port 
unification:

commit efc2f9a8a100511399309a50a33b46ff099f3454
Author: Ralf Baechle <ralf@...ux-mips.org>
Date:   Mon Jul 21 03:03:15 2003 +0000

    Unify tlb-r4k.c.

-- at which point tlb-r3k.c wasn't updated accordingly.  The condition was 
eventually removed from arch/mips/mm/tlb-r4k.c in 2.4 too with:

commit fd8917812546ef2278e006237a7cc38497bfbf52
Author: Ralf Baechle <ralf@...ux-mips.org>
Date:   Thu Nov 25 22:18:38 2004 +0000

    Try to glue the hazard avoidance stuff the same way it was done for
    2.6 before the TLB synthesizer.  Previously the code for some CPUs
    such as the RM9000 was completly bogus nonsense ...  I guess we may
    eventually want to backport the tlb synthesizer to 2.4 once the dust
    has settled.

And arch/mips64/mm/tlb-r4k.c was introduced with:

commit 49be6d9f37bbac0a0c413a2a63dcf7cc497d144a
Author: Ralf Baechle <ralf@...ux-mips.org>
Date:   Wed Jul 24 16:12:03 2002 +0000

    Random 64-bit updates and fixes.

and never had the condition in the first place.

 We do have full 2.6 GIT history, beyond 2.6.12.  It seems cut off at 
1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 for some operations, but not 
plain `git log' for example, and I was able to peek beyond that by 
checking out the immediately preceding commit, that is 
66f0a432564b5f0ebf632263ceac84a10a99de09:

commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2
Author: Linus Torvalds <torvalds@...970.osdl.org>
Date:   Sat Apr 16 15:20:36 2005 -0700

    Linux-2.6.12-rc2

    Initial git repository build. I'm not bothering with the full history,
    even though we have it. We can create a separate "historical" git
    archive of that later if we want to, and in the meantime it's about
    3.2GB when imported into git - space that would just make the early
    git days unnecessarily complicated, when we don't have a lot of good
    infrastructure for it.

    Let it rip!

commit 66f0a432564b5f0ebf632263ceac84a10a99de09
Author: Ralf Baechle <ralf@...ux-mips.org>
Date:   Fri Apr 15 14:40:46 2005 +0000

    Add resource managment.

There may be a better way, I don't claim GIT expertise.

  Maciej
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