linux-kernel - Re: [PATCH 1/2] clone: Declare do

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Date:	Wed, 23 Sep 2015 13:43:20 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Petr Mladek <pmladek@...e.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Thiago Macieira <thiago.macieira@...el.com>,
	Jiri Kosina <jkosina@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] clone: Declare do_fork() only when available

On Wed, 23 Sep 2015 13:59:28 +0200 Petr Mladek <pmladek@...e.com> wrote:

> The commit 3033f14ab78c32687 ("clone: support passing tls argument via C
> rather than pt_regs magic") introduced _do_fork() that allowed to pass
> @tls parameter.
> 
> The old do_fork() is defined only for architectures that are not ready
> to use this way and do not define HAVE_COPY_THREAD_TLS. But the function
> is always declared in linux/sched.h.
> 
> This patch hides the declaration when the function is not defined.
> 
> ..
>
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2618,7 +2618,9 @@ extern int do_execveat(int, struct filename *,
>  		       const char __user * const __user *,
>  		       int);
>  extern long _do_fork(unsigned long, unsigned long, unsigned long, int __user *, int __user *, unsigned long);
> +#ifndef CONFIG_HAVE_COPY_THREAD_TLS
>  extern long do_fork(unsigned long, unsigned long, unsigned long, int __user *, int __user *);
> +#endif

Generally we avoid adding such ifdefs.

Downside: a compile-time error is turned into a link-time error.  This
inconveniences a small number of people a small number of times.

Upside: cleaner-looking header files.  This benefits a larger number of
people a larger number of times.

So it's a good tradeoff, I think.

Now there may be some special reason why do_fork() is special.  If so,
you owe us a new changelog ;)

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