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SubjectRe: [PATCH -next] x86/mm/pat: fix a data race in cpa_inc_4k_install
On Tue, 21 Jan 2020 at 08:27, Borislav Petkov <bp@alien8.de> wrote:
>
> On Mon, Jan 20, 2020 at 11:12:00PM -0500, Qian Cai wrote:
> > diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> > index 20823392f4f2..31e4a73ae70e 100644
> > --- a/arch/x86/mm/pat/set_memory.c
> > +++ b/arch/x86/mm/pat/set_memory.c
> > @@ -128,7 +128,7 @@ static inline void cpa_inc_2m_checked(void)
> >
> > static inline void cpa_inc_4k_install(void)
> > {
> > - cpa_4k_install++;
> > + WRITE_ONCE(cpa_4k_install, READ_ONCE(cpa_4k_install) + 1);

As I said in my email that you also copied to the message, this is
just a stats counter. For the general case, I think we reached
consensus that such accesses should intentionally remain data races:
https://lore.kernel.org/linux-fsdevel/CAHk-=wg5CkOEF8DTez1Qu0XTEFw_oHhxN98bDnFqbY7HL5AB2g@mail.gmail.com/T/#u

Either you can use the data_race() macro, making this
'data_race(cpa_4k_install++)' -- this effectively documents the
intentional data race --

or just blacklist the entire file by putting
KCSAN_SANITIZE_set_memory.o := n
into the Makefile.

[ Note that there are 2 more ways to blacklist:
- __no_kcsan function attribute, for blacklisting entire functions.
- KCSAN_SANITIZE :=n in the Makefile, blacklisting all compilation
units in the Makefile. ]

I leave it to you what makes more sense. I don't know if there are
other data races lurking here, since cpa_4k_install is not the only
stats counter.

> > }
> >
> > static inline void cpa_inc_lp_sameprot(int level)
> > --
>
> Fix a data race, says your subject?
>
> If it had to be honest, it probably should say "Make the code ugly
> because the next tool can't handle it".
>
> Frankly, I'm not a fan of all this "change the kernel to fix the tool"
> attitude.

I commented on better ways dealing with this, since this is just stats counting.

Thanks,
-- Marco

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