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Date:   Mon, 28 Sep 2020 09:32:02 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Chengming Zhou <zhouchengming@bytedance.com>
Cc:     linux-kernel@vger.kernel.org, pmladek@suse.com,
        sergey.senozhatsky@gmail.com, rostedt@goodmis.org,
        mingo@redhat.com, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        bsegall@google.com, mgorman@suse.de, songmuchun@bytedance.com
Subject: Re: [PATCH 2/2] sched: mark PRINTK_DEFERRED_CONTEXT_MASK in
 __schedule()
Message-ID: <20200928073202.GA2611@hirez.programming.kicks-ass.net>
References: <20200927161130.33172-1-zhouchengming@bytedance.com>
 <20200927161130.33172-2-zhouchengming@bytedance.com>
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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@bytedance.com>
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  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.