linux-kernel - Re: [PATCH] Revert "dynamic_debug: Remove unnecessary _

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Message-ID: <59b3e2aac1388209d168a31294dfd2b1f7d21513.camel@perches.com>
Date:   Mon, 11 May 2020 11:54:12 -0700
From:   Joe Perches <joe@...ches.com>
To:     Elliot Berman <eberman@...eaurora.org>,
        Jason Baron <jbaron@...mai.com>
Cc:     Trilok Soni <tsoni@...eaurora.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Revert "dynamic_debug: Remove unnecessary __used"

On Mon, 2020-05-11 at 11:26 -0700, Elliot Berman wrote:
> This reverts commit c0d2af637863940b1a4fb208224ca7acb905c39f.
> 
> Some compilers[1] may break dynamic_hex_dump by optimizing the
> DYNAMIC_DEBUG_BRANCH check inside __dynamic_func_call_no_desc and completely
> removing the dynamic debug metadata. Thus, there is no dynamic_debug control
> site to enable or disable. The if condition is optimized away based on the
> initial value of flags (i.e. if DEBUG macro is present).
> 
> [1]: This behavior is present on [aarch64-gnu-linux-gcc (Ubuntu/Linaro
> 5.4.0-6ubuntu1~16.04.9) 5.4.0 20160609] and [Android Common Kernel
> toolchain: "Android (6051079 based on r370808) clang version
> 10.0.1"] using arm64 default defconfig + DYNAMIC_DEBUG enabled.
> 
> Change-Id: I28e9b86088eee5d5ed2384fbcea2ac2e7337a559
> Signed-off-by: Elliot Berman <eberman@...eaurora.org>
> ---

Change-Id: probably isn't necessary.

Is there a separate mechanism possible to avoid bloating the
otherwise unused content?

>  include/linux/dynamic_debug.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/dynamic_debug.h b/include/linux/dynamic_debug.h
> index abcd5fd..7518db4f 100644
> --- a/include/linux/dynamic_debug.h
> +++ b/include/linux/dynamic_debug.h
> @@ -79,7 +79,7 @@ void __dynamic_ibdev_dbg(struct _ddebug *descriptor,
>  			 const char *fmt, ...);
>  
>  #define DEFINE_DYNAMIC_DEBUG_METADATA(name, fmt)		\
> -	static struct _ddebug  __aligned(8)			\
> +	static struct _ddebug __used __aligned(8)		\
>  	__attribute__((section("__verbose"))) name = {		\
>  		.modname = KBUILD_MODNAME,			\
>  		.function = __func__,				\