Messages in this thread Patch in this message | | | From | Marco Elver <> | Date | Tue, 21 Jan 2020 16:19:22 +0100 | Subject | Re: [PATCH -next] x86/mm/pat: silence a data race in cpa_4k_install |
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On Tue, 21 Jan 2020 at 16:15, Qian Cai <cai@lca.pw> 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@google.com> > Signed-off-by: Qian Cai <cai@lca.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 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) >
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