linux-kernel - Re: [PATCH -next] x86/mm/pat: mark an intentional data race

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Date:   Wed, 11 Mar 2020 12:04:47 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Qian Cai <cai@....pw>, tglx@...utronix.de, mingo@...hat.com,
        dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
        elver@...gle.com, x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next] x86/mm/pat: mark an intentional data race

On Wed, Mar 11, 2020 at 05:17:56PM +0100, Borislav Petkov wrote:
> + Paul.
> 
> On Mon, Feb 10, 2020 at 09:10:16AM -0500, Qian Cai wrote:
> > cpa_4k_install could be accessed concurrently as noticed by KCSAN,
> > 
> > read to 0xffffffffaa59a000 of 8 bytes by interrupt on cpu 7:
> > cpa_inc_4k_install arch/x86/mm/pat/set_memory.c:131 [inline]
> > __change_page_attr+0x10cf/0x1840 arch/x86/mm/pat/set_memory.c:1514
> > __change_page_attr_set_clr+0xce/0x490 arch/x86/mm/pat/set_memory.c:1636
> > __set_pages_np+0xc4/0xf0 arch/x86/mm/pat/set_memory.c:2148
> > __kernel_map_pages+0xb0/0xc8 arch/x86/mm/pat/set_memory.c:2178
> > kernel_map_pages include/linux/mm.h:2719 [inline] <snip>
> > 
> > write to 0xffffffffaa59a000 of 8 bytes by task 1 on cpu 6:
> > cpa_inc_4k_install arch/x86/mm/pat/set_memory.c:131 [inline]
> > __change_page_attr+0x10ea/0x1840 arch/x86/mm/pat/set_memory.c:1514
> > __change_page_attr_set_clr+0xce/0x490 arch/x86/mm/pat/set_memory.c:1636
> > __set_pages_p+0xc4/0xf0 arch/x86/mm/pat/set_memory.c:2129
> > __kernel_map_pages+0x2e/0xc8 arch/x86/mm/pat/set_memory.c:2176
> > kernel_map_pages include/linux/mm.h:2719 [inline] <snip>
> > 
> > Both accesses are due to the same "cpa_4k_install++" in
> > cpa_inc_4k_install. A data race here could be potentially undesirable:
> > depending on compiler optimizations or how x86 executes a non-LOCK'd
> > increment, it may lose increments, corrupt the counter, etc. Since this
> > counter only seems to be used for printing some stats, this data race
> > itself is unlikely to cause harm to the system though. Thus, mark this
> > intentional data race using the data_race() marco.
> > 
> > Suggested-by: Macro Elver <elver@...gle.com>
> > Signed-off-by: Qian Cai <cai@....pw>
> > ---
> >  arch/x86/mm/pat/set_memory.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> > index c4aedd00c1ba..ea0b6df950ee 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++;
> > +	data_race(cpa_4k_install++);
> >  }
> >  
> >  static inline void cpa_inc_lp_sameprot(int level)
> > -- 
> 
> Acked-by: Borislav Petkov <bp@...e.de>

Applied, thank you both!

							Thanx, Paul