linux-kernel - Re: [PATCH v2 05/11] sched,livepatch: Use wake_up_if

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Date:   Thu, 14 Oct 2021 12:42:35 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Vasily Gorbik <gor@...ux.ibm.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Petr Mladek <pmladek@...e.com>, Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        joe.lawrence@...hat.com,
        Frédéric Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Heiko Carstens <hca@...ux.ibm.com>, svens@...ux.ibm.com,
        sumanthk@...ux.ibm.com, live-patching@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [PATCH v2 05/11] sched,livepatch: Use wake_up_if_idle()

On Wed, Oct 13, 2021 at 09:37:01PM +0200, Arnd Bergmann wrote:
> On Wed, Sep 29, 2021 at 6:10 PM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > Make sure to prod idle CPUs so they call klp_update_patch_state().
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >  kernel/livepatch/transition.c |    5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > --- a/kernel/livepatch/transition.c
> > +++ b/kernel/livepatch/transition.c
> > @@ -413,8 +413,11 @@ void klp_try_complete_transition(void)
> >         for_each_possible_cpu(cpu) {
> >                 task = idle_task(cpu);
> >                 if (cpu_online(cpu)) {
> > -                       if (!klp_try_switch_task(task))
> > +                       if (!klp_try_switch_task(task)) {
> >                                 complete = false;
> > +                               /* Make idle task go through the main loop. */
> > +                               wake_up_if_idle(cpu);
> > +                       }
> 
> This caused a build regression on non-SMP kernels:

:-(

> x86_64-linux-ld: kernel/livepatch/transition.o: in function
> `klp_try_complete_transition':
> transition.c:(.text+0x106e): undefined reference to `wake_up_if_idle'
> 
> Maybe add a IS_ENABLED(CONFIG_SMP) check to one of the if() conditions?

I'll just add a stub for that function I think. Let me rebase the thing
before I push more crap ontop..