linux-kernel - Re: Using __always

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Message-ID: <e77e6745b60629315b1b4f0e4fddd4bf@codeaurora.org>
Date:   Wed, 14 Jun 2017 17:50:26 -0700
From:   Sodagudi Prasad <psodagud@...eaurora.org>
To:     Will Deacon <will.deacon@....com>
Cc:     catalin.marinas@....com, mingo@...nel.org, peterz@...radead.org,
        mark.rutland@....com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: Using __always_inline attribute

On 2017-06-14 15:33, Sodagudi Prasad wrote:
> On 2017-06-14 03:06, Will Deacon wrote:
>> Hi Prasad,
>> 
>> On Tue, Jun 13, 2017 at 03:39:37PM -0700, Sodagudi Prasad wrote:
>>> With a variant of a CLANG(based on 4.0) following errors observed on 
>>> Linux
>>> 4.12-rc5 tag.
>> 
>> How are you building the kernel with clang, btw? I'd be keen to try 
>> the same
>> thing, but I thought the LLVMLinux project was largely dead at his 
>> point. Do
>> you still need build system patches, or is this now integrated with 
>> Kbuild?
> Hi Will,
> Trying clang patches from below tree -
> https://android.googlesource.com/kernel/common/+/experimental/android-4.4-llvm
> 
>> 
>>> net/built-in.o: In function `__xchg_mb':
>>> arch/arm64/include/asm/cmpxchg.h:99: \
>>>                     undefined reference to `__compiletime_assert_99'
>>> arch/arm64/include/asm/cmpxchg.h:99: \
>>>                     undefined reference to `__compiletime_assert_99
>>> 
>>> Clang does not seems to be marking this macro as inline and causing 
>>> above
>>> compilation issue due to BUILD_BUG().
>> 
>> The only BUILD_BUG I see around here is if the size parameter (which 
>> is
>> calculated using sizeof) is not known to be 1,2,4 or 8 at compile 
>> time. It
>> would be interesting to know which call site is confusing clang in 
>> this way
>> and what code is actually being emitted here.
>> 
>> If it's just that __xchg_mb isn't being inlined into the 
>> __xchg_wrapper
>> macro, then your patch should be ok, but I'd like to be sure it's not
>> something else. I'm also surprised you haven't see this crop up in 
>> other
>> places.
> After digging further, we observed that inline definition was changed
> recently and causing this issue.
> Here is missing part of inline macro definition 
> __attribute__((unused)).
> 
> Commit abb2ea7dfd82 ("compiler, clang: suppress warning for unused
> static inline functions") have redefined the inline macro as below
> #define inline inline __attribute__((unused))
> 
> But actual definition of inline macro defined compiler-gcc.h file as
> shown below.
> #define inline          inline          __attribute__((always_inline)) 
> notrace
> 
> As always_inline attribute is missing in inline definition, compile
> may not inline macros __xchg_mb in
> arch/arm64/include/asm/cmpxchg.h file and leading to error.
> 
> Some compilers may not honor inline as inline if always_inline
> attribute is removed because of
> -inline-threshold compiler options.
> 
> Here is the change to fix this issue-
> diff --git a/include/linux/compiler-clang.h 
> b/include/linux/compiler-clang.h
> index d614c5e..a0e6433 100644
> --- a/include/linux/compiler-clang.h
> +++ b/include/linux/compiler-clang.h
> @@ -22,4 +22,4 @@
>   * directives.  Suppress the warning in clang as well.
>   */
>  #undef inline
> -#define inline inline __attribute__((unused)) notrace
> +#define inline __attribute__((always_inline)) __attribute__((unused)) 
> notrace
> 
> -Thanks, Prasad

Hi Will,

Here is the proper patch -
     compiler, clang: Add always_inline attribute to inline

     Commit abb2ea7dfd82 ("compiler, clang: suppress warning for unused
     static inline functions") re-defining the 'inline' macro but
     __attribute__((always_inline)) is missing. Some compilers may
     not honor inline hint if always_iniline attribute is not there.
     So add always_inline attribute to inline as done by
     compiler-gcc.h file.

diff --git a/include/linux/compiler-clang.h 
b/include/linux/compiler-clang.h
index d614c5e..400b0cd 100644
--- a/include/linux/compiler-clang.h
+++ b/include/linux/compiler-clang.h
@@ -22,4 +22,4 @@
   * directives.  Suppress the warning in clang as well.
   */
  #undef inline
-#define inline inline __attribute__((unused)) notrace
+#define inline inline __attribute__((always_inline)) 
__attribute__((unused)) notrace

-Thanks, Prasad

>> 
>> Will

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