linux-kernel - Re: [PATCH 3/6] tracing: Wrap section comparison in tracer_alloc_buffers with COMPARE

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Message-ID: <20200219093445.386f1c09@gandalf.local.home>
Date:   Wed, 19 Feb 2020 09:34:45 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...hat.com>,
        Jason Baron <jbaron@...mai.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH 3/6] tracing: Wrap section comparison in
 tracer_alloc_buffers with COMPARE_SECTIONS

On Tue, 18 Feb 2020 21:54:20 -0700
Nathan Chancellor <natechancellor@...il.com> wrote:

> Clang warns:
> 
> ../kernel/trace/trace.c:9335:33: warning: array comparison always
> evaluates to true [-Wtautological-compare]
>         if (__stop___trace_bprintk_fmt != __start___trace_bprintk_fmt)
>                                        ^
> 1 warning generated.
> 
> These are not true arrays, they are linker defined symbols, which are
> just addresses so there is not a real issue here. Use the
> COMPARE_SECTIONS macro to silence this warning by casting the linker
> defined symbols to unsigned long, which keeps the logic the same.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/765
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> ---
>  kernel/trace/trace.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index c797a15a1fc7..e1f3b16e457b 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -9332,7 +9332,7 @@ __init static int tracer_alloc_buffers(void)
>  		goto out_free_buffer_mask;
>  
>  	/* Only allocate trace_printk buffers if a trace_printk exists */
> -	if (__stop___trace_bprintk_fmt != __start___trace_bprintk_fmt)
> +	if (COMPARE_SECTIONS(__stop___trace_bprintk_fmt, !=, __start___trace_bprintk_fmt))

Sorry, but this is really ugly and unreadable. Please find some other
solution to fix this.

NAK-by: Steven Rostedt

-- Steve

>  		/* Must be called before global_trace.buffer is allocated */
>  		trace_printk_init_buffers();
>