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

[Kohli, Gaurav]

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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