Re: [PATCH 3/5] sched: Detect call to schedule from critical entry code

[Peter Zijlstra]

On Mon, Oct 05, 2020 at 12:49:17PM +0200, Frederic Weisbecker wrote:
> Detect calls to schedule() between user_enter() and user_exit(). Those
> are symptoms of early entry code that either forgot to protect a call
> to schedule() inside exception_enter()/exception_exit() or, in the case
> of HAVE_CONTEXT_TRACKING_OFFSTACK, enabled interrupts or preemption in
> a wrong spot.
> 
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Phil Auld <pauld@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---
>  kernel/sched/core.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 2d95dc3f4644..d31a79e073e3 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4295,6 +4295,7 @@ static inline void schedule_debug(struct task_struct *prev, bool preempt)
>  		preempt_count_set(PREEMPT_DISABLED);
>  	}
>  	rcu_sleep_check();
> +	WARN_ON_ONCE(ct_state() == CONTEXT_USER);

	SCHED_WARN_ON() ?

No point in unconditionally polluting that path. Although, per MeL, we
should probably invest in CONFIG_SCHED_DEBUG_I_MEANS_IT :/