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Subject: Re: [PATCH v1] kthread/smpboot: Serialize kthread parking against
 wakeup
To:     Oleg Nesterov <oleg@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     tglx@linutronix.de, mpe@ellerman.id.au, mingo@kernel.org,
        bigeasy@linutronix.de, linux-kernel@vger.kernel.org,
        linux-arm-msm@vger.kernel.org,
        Neeraj Upadhyay <neeraju@codeaurora.org>,
        Will Deacon <will.deacon@arm.com>
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From:   "Kohli, Gaurav" <gkohli@codeaurora.org>
Message-ID: <ea17c818-cd54-898c-6e6f-d17c1708115f@codeaurora.org>
Date:   Tue, 5 Jun 2018 23:51:18 +0530
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Hi,

Just for info , the patch that I have shared earlier with pi_lock 
approach has been tested since last one month and no issue has been 
observed,

https://lkml.org/lkml/2018/4/25/189

Can we take this if it looks good?

Regards
Gaurav

On 6/5/2018 10:05 PM, Oleg Nesterov wrote:
> On 06/05, Peter Zijlstra wrote:
>>
>> On Tue, Jun 05, 2018 at 05:22:12PM +0200, Peter Zijlstra wrote:
>>
>>>> OK, but __kthread_parkme() can be preempted before it calls schedule(), so the
>>>> caller still can be migrated? Plus kthread_park_complete() can be called twice.
>>>
>>> Argh... I forgot TASK_DEAD does the whole thing with preempt_disable().
>>> Let me stare at that a bit.
>>
>> This should ensure we only ever complete when we read PARKED, right?
>>
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 8d59b259af4a..e513b4600796 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -2641,7 +2641,7 @@ prepare_task_switch(struct rq *rq, struct task_struct *prev,
>>    * past. prev == current is still correct but we need to recalculate this_rq
>>    * because prev may have moved to another CPU.
>>    */
>> -static struct rq *finish_task_switch(struct task_struct *prev)
>> +static struct rq *finish_task_switch(struct task_struct *prev, bool preempt)
>>   	__releases(rq->lock)
>>   {
>>   	struct rq *rq = this_rq();
>> @@ -2674,7 +2674,7 @@ static struct rq *finish_task_switch(struct task_struct *prev)
>>   	 *
>>   	 * We must observe prev->state before clearing prev->on_cpu (in
>>   	 * finish_task), otherwise a concurrent wakeup can get prev
>> -	 * running on another CPU and we could rave with its RUNNING -> DEAD
>> +	 * running on another CPU and we could race with its RUNNING -> DEAD
>>   	 * transition, resulting in a double drop.
>>   	 */
>>   	prev_state = prev->state;
>> @@ -2720,7 +2720,8 @@ static struct rq *finish_task_switch(struct task_struct *prev)
>>   			break;
>>
>>   		case TASK_PARKED:
>> -			kthread_park_complete(prev);
>> +			if (!preempt)
>> +				kthread_park_complete(prev);
> 
> 
> Yes, but this won't fix the race decribed by Kohli...
> 
> Plus this complicates the schedule() paths for the very special case, and to me
> it seems that all this kthread_park/unpark logic needs some serious cleanups...
> 
> Not that I can suggest something better right now.
> 
> Oleg.
> 
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