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

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Date:   Fri, 06 Mar 2020 09:16:19 -0500
From:   Qian Cai <cai@....pw>
To:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de
Cc:     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 Mon, 2020-02-10 at 09:10 -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.

Borislav or any other maintainers, can you take a look at this patch when you
had a chance?

> 
> 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)