Re: [PATCH v3 0/2] Unify uapi bitsperlong.h

[Ian Rogers <irogers@google.com>]

On Thu, Jun 22, 2023 at 8:10 AM Arnd Bergmann <arnd@arndb.de> wrote:
>
> On Thu, Jun 22, 2023, at 16:13, Tiezhu Yang wrote:
> > v3:
> >   -- Check the definition of __BITS_PER_LONG first at
> >      the beginning of uapi/asm-generic/bitsperlong.h
> >

Thanks for doing this cleanup! I just wanted to report an issue I ran
into with building the Linux perf tool. The header guard in:
tools/include/asm-generic/bitsperlong.h
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/include/asm-generic/bitsperlong.h

Caused an issue with building:
tools/perf/util/cs-etm.c
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/perf/util/cs-etm.c

The issue was that cs-etm.c would #include a system header, which
would transitively include a header with the same header guard. This
led to the tools/include/asm-generic/bitsperlong.h being ignored and
the compilation of tools/perf/util/cs-etm.c failing due to a missing
define. My local workaround is:

```
diff --git a/tools/include/asm-generic/bitsperlong.h
b/tools/include/asm-generic/bitsperlong.h
index 2093d56ddd11..88508a35cb45 100644
--- a/tools/include/asm-generic/bitsperlong.h
+++ b/tools/include/asm-generic/bitsperlong.h
@@ -1,6 +1,6 @@
/* SPDX-License-Identifier: GPL-2.0 */
-#ifndef __ASM_GENERIC_BITS_PER_LONG
-#define __ASM_GENERIC_BITS_PER_LONG
+#ifndef __LINUX_TOOLS_ASM_GENERIC_BITS_PER_LONG
+#define __LINUX_TOOLS_ASM_GENERIC_BITS_PER_LONG
#include <uapi/asm-generic/bitsperlong.h>
@@ -21,4 +21,4 @@
#define small_const_nbits(nbits) \
(__builtin_constant_p(nbits) && (nbits) <= BITS_PER_LONG && (nbits) > 0)
-#endif /* __ASM_GENERIC_BITS_PER_LONG */
+#endif /* __LINUX_TOOLS_ASM_GENERIC_BITS_PER_LONG */
```

I'm not sure if a wider fix is necessary for this, but I thought it
worthwhile to report that there are potential issues. I don't think we
can use #pragma once, as an alternative to header guards, to avoid
this kind of name collision.

Thanks,
Ian


> > v2:
> >   -- Check __CHAR_BIT__ and __SIZEOF_LONG__ rather than
> >      __aarch64__, __riscv, __loongarch__, thanks Ruoyao
> >   -- Update the code comment and commit message
> >
> > v1:
> >   -- Rebase on 6.4-rc6
> >   -- Only unify uapi bitsperlong.h for arm64, riscv and loongarch
> >   -- Remove uapi bitsperlong.h of hexagon and microblaze in a new patch
> >
> > Here is the RFC patch:
> > https://lore.kernel.org/linux-arch/1683615903-10862-1-git-send-email-yangtiezhu@loongson.cn/
>
> I've applied these to the asm-generic tree now
>
> Thanks,
>
>    Arnd