linux-kernel - Re: [PATCH 2/3] static_call: Align static_call_is

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Date:   Fri, 19 Mar 2021 15:13:54 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     x86@...nel.org, jpoimboe@...hat.com, jbaron@...mai.com,
        rostedt@...dmis.org, ardb@...nel.org, linux-kernel@...r.kernel.org,
        sumit.garg@...aro.org, oliver.sang@...el.com, jarkko@...nel.org
Subject: Re: [PATCH 2/3] static_call: Align static_call_is_init() patching
 condition

On Fri, Mar 19, 2021 at 02:31:19PM +0100, Rasmus Villemoes wrote:
> On 18/03/2021 12.31, Peter Zijlstra wrote:
> > The intent is to avoid writing init code after init (because the text
> > might have been freed). The code is needlessly different between
> > jump_label and static_call and not obviously correct.
> > 
> > The existing code relies on the fact that the module loader clears the
> > init layout, such that within_module_init() always fails, while
> > jump_label relies on the module state which is more obvious and
> > matches the kernel logic.
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >  kernel/static_call.c |   14 ++++----------
> >  1 file changed, 4 insertions(+), 10 deletions(-)
> > 
> > --- a/kernel/static_call.c
> > +++ b/kernel/static_call.c
> > @@ -149,6 +149,7 @@ void __static_call_update(struct static_
> >  	};
> >  
> >  	for (site_mod = &first; site_mod; site_mod = site_mod->next) {
> > +		bool init = system_state < SYSTEM_RUNNING;
> 
> I recently had occasion to look at whether that would be a suitable
> condition for knowing whether __init stuff was gone, but concluded that
> it's not. Maybe I'm wrong. init/main.c:

Ha, me too:

 https://lkml.kernel.org/r/YFMToXI/3qjlMur4@hirez.programming.kicks-ass.net

and I share your concern.

> Dunno, probably overkill, but perhaps we could have an atomic_t (or
> refcount, whatever) init_ref inited to 1, with init_ref_get() doing an
> inc_unless_zero, and iff you get a ref, you're free to call (/patch)
> __init functions and access __initdata, but must do init_ref_put(), with
> PID1 dropping its initial ref and waiting for it to drop to 0 before
> doing the *free_initmem() calls.

I'd as soon simply add another SYSTEM state. That way we don't have to
worry about who else looks at RUNNING for what etc..