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

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Date:   Mon, 22 Mar 2021 17:59:08 +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 03:40:46PM +0100, Rasmus Villemoes wrote:
> On 19/03/2021 15.13, Peter Zijlstra wrote:
> 
> >> 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..
> 
> I don't understand. How would that solve the
> 
> PID1                           PIDX
>                                ok = system_state < INIT_MEM_GONE;
> system_state = INIT_MEM_GONE;
> free_initmem();
> system_state = RUNNING;
>                                if (ok)
>                                    poke init mem
> 
> race?

Argh, I meant to put it before SMP bringup, but then still have to run
the smp_init calls :/

N/m, you're quite right.