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

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Message-ID: <20091001203905.GD2660@redhat.com>
Date:	Thu, 1 Oct 2009 16:39:05 -0400
From:	Jason Baron <jbaron@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
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

On Thu, Oct 01, 2009 at 01:20:03PM +0200, Ingo Molnar wrote:
> * 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.
> 

The specific case I hit was in modifying code in arch_kdebugfs_init()
which is '__init' after the system was up and running. The tracepoint is
in 'kmalloc()' which is marked as __always_inline.


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

I was trying to make sure that tracepoints in init-text were honored.

> 
> 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.
> 

hmmm....indeed, kernel_text_address() does do what I want here (I must
have mis-read its definition). Although, I'm not sure there isn't a race
here betweeen freeing the init sections and possibly updating them. For
modules, there is no race since the module init free code takes the
module_mutex, and I do as well in this code...

I've now also tested this code on 32-bit x86 system, and it seems to
perform nicely. I'm seeing a 15 cycle improvement per tracepoint.

I've based the text section updating on text_poke_fixup(), which has
recently come into question about safety of cross modifying code. I
could rebase my patches back to use stop_machine()? I guess I'm looking
for some advice on how to proceed here.

thanks,

-Jason




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