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From:   Arnd Bergmann <arnd@kernel.org>
Date:   Wed, 13 Oct 2021 21:37:01 +0200
Message-ID: <CAK8P3a0N-ZuSEZyw5ub1vr3VP2Bdoa2Wq=No1gad+SyqquQXfw@mail.gmail.com>
Subject: Re: [PATCH v2 05/11] sched,livepatch: Use wake_up_if_idle()
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Vasily Gorbik <gor@linux.ibm.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Jiri Kosina <jikos@kernel.org>,
        Miroslav Benes <mbenes@suse.cz>,
        Petr Mladek <pmladek@suse.com>, Ingo Molnar <mingo@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        joe.lawrence@redhat.com,
        =?UTF-8?B?RnLDqWTDqXJpYyBXZWlzYmVja2Vy?= <fweisbec@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Heiko Carstens <hca@linux.ibm.com>, svens@linux.ibm.com,
        sumanthk@linux.ibm.com, live-patching@vger.kernel.org,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        "the arch/x86 maintainers" <x86@kernel.org>
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On Wed, Sep 29, 2021 at 6:10 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> Make sure to prod idle CPUs so they call klp_update_patch_state().
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.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?

        Arnd