Re: [PATCH -next] x86/mm/pat: silence a data race in cpa_4k_install

[Marco Elver]

On Tue, 21 Jan 2020 at 16:15, Qian Cai <cai@xxxxxx> wrote:
>
> Macro Elver mentioned,
>
> "Yes. I was finally able to reproduce this data race on linux-next (my
> system doesn't crash though, maybe not enough cores?). Here is a trace
> with line numbers:
>
> 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. Now you can see that a data race here could be
> potentially undesirable: depending on compiler optimizations or how
> x86 executes a non-LOCK'd increment, you 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."

Thank you for the patch!

Could you remove the verbatim copy of my email? Maybe something like:

"Increments to cpa_4k_install may happen concurrently, as detected by KCSAN:

<....... the stack traces ......>

Since the counter is only used to count stats, a data race will not be
harmful, thus we mark it as an intentional data race with the
'data_race()' macro.

Otherwise, this may generate a lot of noise on a debug kernel with
debug_pagealloc
with KCSAN enabled which could render the system unusable."

Thanks,
-- Marco

> This will generate a lot of noise on a debug kernel with debug_pagealloc
> with KCSAN enabled which could render the system unusable. Silence it by
> using the data_race() macro.
>
> Suggested-by: Macro Elver <elver@xxxxxxxxxx>
> Signed-off-by: Qian Cai <cai@xxxxxx>
> ---
>  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 20823392f4f2..a5c35e57905e 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)
> --
> 2.21.0 (Apple Git-122.2)
>