linux-kernel - Re: [patch V2 17/17] sched: Enable might_sleep() and smp_processor

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Date:   Wed, 17 May 2017 12:02:16 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [patch V2 17/17] sched: Enable might_sleep() and
 smp_processor_id() checks early

On Tue, May 16, 2017 at 08:42:48PM +0200, Thomas Gleixner wrote:
> might_sleep() and smp_processor_id() checks are enabled after the boot
> process is done. That hides bugs in the smp bringup and driver
> initialization code.
> 
> Enable it right when the scheduler starts working, i.e. when init task and
> kthreadd have been created and right before the idle task enables
> preemption.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

Modulo Steve's comments, FWIW:

Acked-by: Mark Rutland <mark.rutland@....com>

Mark.

> ---
>  init/main.c            |   10 ++++++++++
>  kernel/sched/core.c    |    4 +++-
>  lib/smp_processor_id.c |    2 +-
>  3 files changed, 14 insertions(+), 2 deletions(-)
> 
> --- a/init/main.c
> +++ b/init/main.c
> @@ -414,6 +414,16 @@ static noinline void __ref rest_init(voi
>  	rcu_read_lock();
>  	kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns);
>  	rcu_read_unlock();
> +
> +	/*
> +	 * Enable might_sleep() and smp_processor_id() checks.
> +	 * They cannot be enabled earlier because with CONFIG_PRREMPT=y
> +	 * kernel_thread() would trigger might_sleep() splats. With
> +	 * CONFIG_PREEMPT_VOLUNTARY=y the init task might have scheduled
> +	 * already, but it's stuck on the kthreadd_done completion.
> +	 */
> +	system_state = SYSTEM_SCHEDULING;
> +
>  	complete(&kthreadd_done);
>  
>  	/*
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6226,8 +6226,10 @@ void ___might_sleep(const char *file, in
>  
>  	if ((preempt_count_equals(preempt_offset) && !irqs_disabled() &&
>  	     !is_idle_task(current)) ||
> -	    system_state != SYSTEM_RUNNING || oops_in_progress)
> +	    system_state == SYSTEM_BOOTING || system_state > SYSTEM_RUNNING ||
> +	    oops_in_progress)
>  		return;
> +
>  	if (time_before(jiffies, prev_jiffy + HZ) && prev_jiffy)
>  		return;
>  	prev_jiffy = jiffies;
> --- a/lib/smp_processor_id.c
> +++ b/lib/smp_processor_id.c
> @@ -28,7 +28,7 @@ notrace static unsigned int check_preemp
>  	/*
>  	 * It is valid to assume CPU-locality during early bootup:
>  	 */
> -	if (system_state != SYSTEM_RUNNING)
> +	if (system_state < SYSTEM_SCHEDULING)
>  		goto out;
>  
>  	/*
> 
>