linux-kernel - Re: [PATCH 2/2] Rewrite jump_label.c to use binary search

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Message-ID: <20100922152833.GD2816@redhat.com>
Date:	Wed, 22 Sep 2010 11:28:34 -0400
From:	Jason Baron <jbaron@...hat.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Andi Kleen <andi@...stfloor.org>, rusty@...tcorp.co.au,
	rostedt@...dmis.com, linux-kernel@...r.kernel.org, mingo@...e.hu,
	hpa@...or.com, tglx@...utronix.de, roland@...hat.com,
	rth@...hat.com, mhiramat@...hat.com, fweisbec@...il.com,
	avi@...hat.com, davem@...emloft.net, vgoyal@...hat.com,
	sam@...nborg.org, tony@...eyournoodle.com,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 2/2] Rewrite jump_label.c to use binary search

On Wed, Sep 22, 2010 at 11:02:50AM -0400, Mathieu Desnoyers wrote:
> * Andi Kleen (andi@...stfloor.org) wrote:
> > 
> > >
> > >>> +	for (; entry < stop && entry->key == key; entry++)
> > >>> +		if (kernel_text_address(entry->code))
> > >>
> > >> This does not work for modules I'm afraid, only for the core kernel. You
> > >> should test for __module_text_address() somewhere.
> > >
> > > I thought it was shared now, but ok.
> > 
> > Double checked. This is ok because kernel_text_address()
> > already checks for modules. You were probably thinking
> > of __kernel_text_address()
> 
> Ah right,
> 
> Although we have another problem:
> 
> __module_text_address() includes module init text, which defeats the
> purpose of the check put in there by Jason.
> 
> So the check works for the core kernel, but not for modules.
> 
> Mathieu
> 

it works for modules too...it does:

struct module *__module_text_address(unsigned long addr)
{
        struct module *mod = __module_address(addr);
        if (mod) {
                /* Make sure it's within the text section. */
                if (!within(addr, mod->module_init, mod->init_text_size)
                    && !within(addr, mod->module_core,
mod->core_text_size))
                        mod = NULL;
        }
        return mod;
}

and then in kernel/module.c we have :


        module_free(mod, mod->module_init);
        mod->module_init = NULL;


So, I was relying on the fact module_init gets set to NULL after the
free happens. However, there a small race there in that the vfree()
happens before module_init() is set to NULL. So that is probably most
easily fixed be wrapping those two lines with the jump_label_mutex.

thanks,

-Jason


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