linux-kernel - Re: [PATCH 1/2] Add for_each

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Date:	Wed, 22 Sep 2010 10:29:31 -0400
From:	Jason Baron <jbaron@...hat.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Andi Kleen <andi@...stfloor.org>, rostedt@...dmis.org,
	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 1/2] Add for_each_module iterator function v2

On Wed, Sep 22, 2010 at 10:14:32AM -0400, Mathieu Desnoyers wrote:
> * Andi Kleen (andi@...stfloor.org) wrote:
> > From: Andi Kleen <ak@...ux.intel.com>
> > 
> > This is a generic function to iterate over all modules.
> > To be used in the next patch.
> > 
> > v2: Use module_mutex instead of preempt disabling
> > 
> > Signed-off-by: Andi Kleen <ak@...ux.intel.com>
> > ---
> >  include/linux/module.h |    1 +
> >  kernel/module.c        |   10 ++++++++++
> >  2 files changed, 11 insertions(+), 0 deletions(-)
> > 
> > diff --git a/include/linux/module.h b/include/linux/module.h
> > index 403ac26..809b6db 100644
> > --- a/include/linux/module.h
> > +++ b/include/linux/module.h
> > @@ -410,6 +410,7 @@ struct module *__module_address(unsigned long addr);
> >  bool is_module_address(unsigned long addr);
> >  bool is_module_percpu_address(unsigned long addr);
> >  bool is_module_text_address(unsigned long addr);
> > +void for_each_module(void (*op)(struct module *, void *arg), void *arg);
> >  
> >  static inline int within_module_core(unsigned long addr, struct module *mod)
> >  {
> > diff --git a/kernel/module.c b/kernel/module.c
> > index eba1341..9e2a561 100644
> > --- a/kernel/module.c
> > +++ b/kernel/module.c
> > @@ -384,6 +384,16 @@ struct module *find_module(const char *name)
> >  }
> >  EXPORT_SYMBOL_GPL(find_module);
> >  
> > +void for_each_module(void (*op)(struct module *, void *arg), void *arg)
> > +{
> > +	struct module *mod;
> > +
> > +	mutex_lock(&module_mutex);
> 
> I'm concerned about the following mutex dependency chain:
> 
> module_mutex -> jump_label_mutex (on module load)

with the new code, this path doesn't exist. No state needs to be updated
at this point.

> jump_label_mutex -> module_mutex (on jump_label_update)
> 
> Actually, I think your module notifier is missing in your version. This
> is why you don't see this dependency problem.
> 

the jump label design in stateless. When a new module is loaded the
module notifier in the tracepoint call will call jump_label_update via
enable_jump_label(). Since the module is on the the module_list before
the notifier, I think this is ok.

thanks,

-Jason
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