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SubjectRe: [PATCH v1] kthread/smpboot: Serialize kthread parking against wakeup
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Date
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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