linux-kernel - Re: [PATCH 1/4] jump label - make init_kernel

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Message-ID: <20091001112003.GA2962@elte.hu>
Date:	Thu, 1 Oct 2009 13:20:03 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jason Baron <jbaron@...hat.com>
Cc:	linux-kernel@...r.kernel.org, mathieu.desnoyers@...ymtl.ca,
	tglx@...utronix.de, rostedt@...dmis.org, ak@...e.de,
	roland@...hat.com, rth@...hat.com, mhiramat@...hat.com
Subject: Re: [PATCH 1/4] jump label - make init_kernel_text() global


* Jason Baron <jbaron@...hat.com> wrote:

> allow usage of init_kernel_text - we need this in jump labeling to 
> avoid attemtpting to patch code that has been freed as in the __init 
> sections

s/attemtpting/attempting

> Signed-off-by: Jason Baron <jbaron@...hat.com>
> ---
>  include/linux/kernel.h |    1 +
>  kernel/extable.c       |    2 +-
>  2 files changed, 2 insertions(+), 1 deletions(-)
> 
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index f61039e..9d3419f 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -295,6 +295,7 @@ extern int get_option(char **str, int *pint);
>  extern char *get_options(const char *str, int nints, int *ints);
>  extern unsigned long long memparse(const char *ptr, char **retptr);
>  
> +extern int init_kernel_text(unsigned long addr);
>  extern int core_kernel_text(unsigned long addr);
>  extern int __kernel_text_address(unsigned long addr);
>  extern int kernel_text_address(unsigned long addr);
> diff --git a/kernel/extable.c b/kernel/extable.c
> index 7f8f263..f6893ad 100644
> --- a/kernel/extable.c
> +++ b/kernel/extable.c
> @@ -52,7 +52,7 @@ const struct exception_table_entry *search_exception_tables(unsigned long addr)
>  	return e;
>  }
>  
> -static inline int init_kernel_text(unsigned long addr)
> +int init_kernel_text(unsigned long addr)
>  {
>  	if (addr >= (unsigned long)_sinittext &&
>  	    addr <= (unsigned long)_einittext)

i'm confused. Later on jump_label_update() does:

+                       if (!(system_state == SYSTEM_RUNNING &&
+                                       (init_kernel_text(iter->code))))
+                               jump_label_transform(iter, type);

which is:

+                       if (system_state != SYSTEM_RUNNING ||
+                                       !init_kernel_text(iter->code)))
+                               jump_label_transform(iter, type);

What is the logic behind that? System going into SYSTEM_RUNNING does not 
coincide with free_initmem() precisely.

Also, do we ever want to patch init-text tracepoints? I think we want to 
stay away from them as much as possible.

It appears to me that what we want here is a straight:

                       if (kernel_text(iter->code))
                               jump_label_transform(iter, type);

Also, maybe a WARN_ONCE(!kernel_text()) - we should never even attempt 
to transform non-patchable code. If yes then we want to know about that 
in a noisy way and not skip it silently.

	Ingo
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