Re: [PATCH 11/12] tools: sync lib/find_bit implementation

Yury Norov
2021-05-10T17:21:37Z

On Mon, May 10, 2021 at 06:44:44PM +0300, Andy Shevchenko wrote:
> +Cc: Rikard
>
> On Mon, May 10, 2021 at 6:31 PM Tetsuo Handa
> <penguin-kernel@i-love.sakura.ne.jp> wrote:
> >
> > Commit eaae7841ba83bb42 ("tools: sync lib/find_bit implementation") broke
> > build of 5.13-rc1 using gcc (GCC) 8.3.1 20190311 (Red Hat 8.3.1-3).
> >
> > DESCEND objtool
> > CC /usr/src/linux/tools/objtool/exec-cmd.o
> > CC /usr/src/linux/tools/objtool/help.o
> > CC /usr/src/linux/tools/objtool/pager.o
> > CC /usr/src/linux/tools/objtool/parse-options.o
> > CC /usr/src/linux/tools/objtool/run-command.o
> > CC /usr/src/linux/tools/objtool/sigchain.o
> > CC /usr/src/linux/tools/objtool/subcmd-config.o
> > LD /usr/src/linux/tools/objtool/libsubcmd-in.o
> > AR /usr/src/linux/tools/objtool/libsubcmd.a
> > CC /usr/src/linux/tools/objtool/arch/x86/special.o
> > In file included from /usr/src/linux/tools/include/linux/kernel.h:8:0,
> > from /usr/src/linux/tools/include/linux/list.h:7,
> > from /usr/src/linux/tools/objtool/include/objtool/arch.h:10,
> > from /usr/src/linux/tools/objtool/include/objtool/check.h:11,
> > from /usr/src/linux/tools/objtool/include/objtool/special.h:10,
> > from arch/x86/special.c:4:
> > /usr/src/linux/tools/include/asm-generic/bitops/find.h: In function 'find_next_bit':
> > /usr/src/linux/tools/include/linux/bits.h:24:21: error: first argument to '__builtin_choose_expr' not a constant
> > (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> > ^
> > /usr/src/linux/tools/include/linux/build_bug.h:16:62: note: in definition of macro 'BUILD_BUG_ON_ZERO'
> > #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))
> > ^
> > /usr/src/linux/tools/include/linux/bits.h:38:3: note: in expansion of macro 'GENMASK_INPUT_CHECK'
> > (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
> > ^
> > /usr/src/linux/tools/include/asm-generic/bitops/find.h:32:17: note: in expansion of macro 'GENMASK'
> > val = *addr & GENMASK(size - 1, offset);
> > ^
> > /usr/src/linux/tools/include/linux/build_bug.h:16:51: error: bit-field '<anonymous>' width not an integer constant
> > #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))
> > ^
> > /usr/src/linux/tools/include/linux/bits.h:24:3: note: in expansion of macro 'BUILD_BUG_ON_ZERO'
> > (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> > ^
> > /usr/src/linux/tools/include/linux/bits.h:38:3: note: in expansion of macro 'GENMASK_INPUT_CHECK'
> > (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
> > ^
> > /usr/src/linux/tools/include/asm-generic/bitops/find.h:32:17: note: in expansion of macro 'GENMASK'
> > val = *addr & GENMASK(size - 1, offset);
> > ^
> > /usr/src/linux/tools/include/asm-generic/bitops/find.h: In function 'find_next_and_bit':
> > /usr/src/linux/tools/include/linux/bits.h:24:21: error: first argument to '__builtin_choose_expr' not a constant
> > (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> > ^
> > /usr/src/linux/tools/include/linux/build_bug.h:16:62: note: in definition of macro 'BUILD_BUG_ON_ZERO'
> > #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))
> > ^
> > /usr/src/linux/tools/include/linux/bits.h:38:3: note: in expansion of macro 'GENMASK_INPUT_CHECK'
> > (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
> > ^
> > /usr/src/linux/tools/include/asm-generic/bitops/find.h:62:27: note: in expansion of macro 'GENMASK'
> > val = *addr1 & *addr2 & GENMASK(size - 1, offset);
> > ^
> > /usr/src/linux/tools/include/linux/build_bug.h:16:51: error: bit-field '<anonymous>' width not an integer constant
> > #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))
> > ^
> > /usr/src/linux/tools/include/linux/bits.h:24:3: note: in expansion of macro 'BUILD_BUG_ON_ZERO'
> > (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> > ^
> > /usr/src/linux/tools/include/linux/bits.h:38:3: note: in expansion of macro 'GENMASK_INPUT_CHECK'
> > (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
> > ^
> > /usr/src/linux/tools/include/asm-generic/bitops/find.h:62:27: note: in expansion of macro 'GENMASK'
> > val = *addr1 & *addr2 & GENMASK(size - 1, offset);
> > ^
> > /usr/src/linux/tools/include/asm-generic/bitops/find.h: In function 'find_next_zero_bit':
> > /usr/src/linux/tools/include/linux/bits.h:24:21: error: first argument to '__builtin_choose_expr' not a constant
> > (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> > ^
> > /usr/src/linux/tools/include/linux/build_bug.h:16:62: note: in definition of macro 'BUILD_BUG_ON_ZERO'
> > #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))
> > ^
> > /usr/src/linux/tools/include/linux/bits.h:38:3: note: in expansion of macro 'GENMASK_INPUT_CHECK'
> > (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
> > ^
> > /usr/src/linux/tools/include/asm-generic/bitops/find.h:90:18: note: in expansion of macro 'GENMASK'
> > val = *addr | ~GENMASK(size - 1, offset);
> > ^
> > /usr/src/linux/tools/include/linux/build_bug.h:16:51: error: bit-field '<anonymous>' width not an integer constant
> > #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))
> > ^
> > /usr/src/linux/tools/include/linux/bits.h:24:3: note: in expansion of macro 'BUILD_BUG_ON_ZERO'
> > (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> > ^
> > /usr/src/linux/tools/include/linux/bits.h:38:3: note: in expansion of macro 'GENMASK_INPUT_CHECK'
> > (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
> > ^
> > /usr/src/linux/tools/include/asm-generic/bitops/find.h:90:18: note: in expansion of macro 'GENMASK'
> > val = *addr | ~GENMASK(size - 1, offset);
> > ^
> > make[5]: *** [/usr/src/linux/tools/objtool/arch/x86/special.o] Error 1
> > make[4]: *** [arch/x86] Error 2
> > make[3]: *** [/usr/src/linux/tools/objtool/objtool-in.o] Error 2
> > make[2]: *** [objtool] Error 2
> > make[1]: *** [tools/objtool] Error 2
> > make: *** [__sub-make] Error 2
> >
> >
> > Applying below diff seems to solve the build failure.
> >
> It will desynchronize this implementation with the mother's one (i.e.
> in bits.h).
>
> > Do we need to use BUILD_BUG_ON_ZERO() here?
>
> Rikard?
>
> >
> > diff --git a/tools/include/linux/bits.h b/tools/include/linux/bits.h
> > index 7f475d59a097..0aba9294f29d 100644
> > --- a/tools/include/linux/bits.h
> > +++ b/tools/include/linux/bits.h
> > @@ -21,8 +21,7 @@
> > #if !defined(__ASSEMBLY__)
> > #include <linux/build_bug.h>
> > #define GENMASK_INPUT_CHECK(h, l) \
> > - (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> > - __builtin_constant_p((l) > (h)), (l) > (h), 0)))
> > + ({ BUILD_BUG_ON(__builtin_constant_p((l) > (h)) && ((l) > (h))); 0; })
> > #else
> > /*
> > * BUILD_BUG_ON_ZERO is not available in h files included from asm files,

As Andy said, we need to sync tools/GENMASK_INPUT_CHECK() with the
kernel version, and if I do this, I have many build failures:

https://pastebin.com/Fan1VLVF

Maybe in this case we should use __GENMASK() to bypass GENMASK_INPUT_CHECK()...
I'll check everything carefully this evening.

> > Also, why the fast path of find_*_bit() functions does not check
> > __builtin_constant_p(offset) as well as small_const_nbits(size), for the fast
> > path fails to catch BUILD_BUG_ON_ZERO() when offset argument is not a constant.
>
> How would this help anything?
>
> If you ask a bit from a bitmap behind the size, what do you expect to get?
>
> And I'm a bit lost here, because I can't imagine the offset being
> constant along with a size of bitmap. What do we want to achieve by
> this? Any examples to better understand the case?

If offset is constant, the existing fast path optimization would work
even better, without any modifications. (Not sure there's an example of
it in the existing codebase.) But even if the offset is not constant,
fast path works quite well - it saves ~1K of .text and improves on
performance:

https://lore.kernel.org/linux-m68k/20210321215457.588554-10-yury.norov@gmail.com/

We don't need to disable the optimization for non-constant offsets,
for sure. If asserts in GENMASK() break build, we should use __GENMASK().

Thanks,
Yury