linux-kernel - Re: [PATCH -v3 6/7] kexec jump: __ftrace_enabled

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Message-ID: <20080812130609.GB3951@redhat.com>
Date:	Tue, 12 Aug 2008 09:06:09 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Huang Ying <ying.huang@...el.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Pavel Machek <pavel@....cz>, nigel@...el.suspend2.net,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Andrew Morton <akpm@...ux-foundation.org>, mingo@...e.hu,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Kexec Mailing List <kexec@...ts.infradead.org>
Subject: Re: [PATCH -v3 6/7] kexec jump: __ftrace_enabled_save/restore

On Tue, Aug 12, 2008 at 11:14:36AM +0800, Huang Ying wrote:
> Add __ftrace_enabled_save/restore, used to disable ftrace for a
> while. Now, this is used by kexec jump, which need a version without
> lock, for general situation, a locked version should be used.
> 
> Signed-off-by: Huang Ying <ying.huang@...el.com>
> 
> ---
>  include/linux/ftrace.h |   21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -98,6 +98,27 @@ static inline void tracer_disable(void)
>  #endif
>  }
>  
> +/* Ftrace disable/restore without lock. Some synchronization mechanism
> + * must be used to prevent ftrace_enabled to be changed between
> + * disable/restore. */
> +static inline int __ftrace_enabled_save(void)
> +{
> +#ifdef CONFIG_FTRACE
> +	int saved_ftrace_enabled = ftrace_enabled;
> +	ftrace_enabled = 0;
> +	return saved_ftrace_enabled;
> +#else
> +	return 0;
> +#endif
> +}
> +
> +static inline void __ftrace_enabled_restore(int enabled)
> +{
> +#ifdef CONFIG_FTRACE
> +	ftrace_enabled = enabled;
> +#endif
> +}
> +
>  #ifdef CONFIG_FRAME_POINTER
>  /* TODO: need to fix this for ARM */
>  # define CALLER_ADDR0 ((unsigned long)__builtin_return_address(0))

I guess steven would like to see a patch which introduces both locked
and lockless versions and with a very good comment explaining in what
kind of unusual situation one can use the lockless version.

Thanks
Vivek
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