Re: [PATCH 2/2] sched: mark PRINTK_DEFERRED_CONTEXT_MASK in __schedule()

[Peter Zijlstra]

On Mon, Sep 28, 2020 at 12:11:30AM +0800, Chengming Zhou wrote:
> The WARN_ON/WARN_ON_ONCE with rq lock held in __schedule() should be
> deferred by marking the PRINTK_DEFERRED_CONTEXT_MASK, or will cause
> deadlock on rq lock in the printk path.

It also shouldn't happen in the first place, so who bloody cares.

> Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> ---
>  kernel/sched/core.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 2d95dc3f4644..81d8bf614225 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4444,6 +4444,7 @@ static void __sched notrace __schedule(bool preempt)
>  	 */
>  	rq_lock(rq, &rf);
>  	smp_mb__after_spinlock();
> +	printk_deferred_enter();
>  
>  	/* Promote REQ to ACT */
>  	rq->clock_update_flags <<= 1;
> @@ -4530,6 +4531,7 @@ static void __sched notrace __schedule(bool preempt)
>  		rq_unlock_irq(rq, &rf);
>  	}
>  
> +	printk_deferred_exit();
>  	balance_callback(rq);
>  }

NAK printk_deferred is an abomination, kill that.