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Subject: Re: [External] Re: [PATCH 2/2] sched: mark
 PRINTK_DEFERRED_CONTEXT_MASK in __schedule()
To:     Peter Zijlstra <peterz@infradead.org>
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
References: <20200927161130.33172-1-zhouchengming@bytedance.com>
 <20200927161130.33172-2-zhouchengming@bytedance.com>
 <20200928073202.GA2611@hirez.programming.kicks-ass.net>
From:   Chengming Zhou <zhouchengming@bytedance.com>
Message-ID: <40ab934e-5b8b-735b-da65-3043efab9fdc@bytedance.com>
Date:   Mon, 28 Sep 2020 16:54:53 +0800
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在 2020/9/28 下午3:32, Peter Zijlstra 写道:
> 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.

Yes, but if our box deadlock just because a WARN_ON_ONCE, we have to
reboot : (

So these WARN_ON_ONCE have BUG_ON effect ?  Or we should change to use
BUG_ON ?

Thanks.

>> 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.