On Wed, Nov 11, 2015 at 08:51:47AM +0900, Byungchul Park wrote: > On Tue, Nov 10, 2015 at 01:16:47PM +0100, Peter Zijlstra wrote: > > On Tue, Nov 10, 2015 at 10:09:05AM +0900, Byungchul Park wrote: > > > On Mon, Nov 09, 2015 at 02:29:14PM +0100, Peter Zijlstra wrote: > > > > On Sat, Oct 24, 2015 at 01:16:21AM +0900, [email protected] wrote: > > > > > +++ b/kernel/sched/core.c > > > > > @@ -1264,6 +1264,8 @@ EXPORT_SYMBOL_GPL(set_cpus_allowed_ptr); > > > > > > > > > > void set_task_cpu(struct task_struct *p, unsigned int new_cpu) > > > > > { > > > > > + unsigned int prev_cpu = task_cpu(p); > > > > > + > > > > > #ifdef CONFIG_SCHED_DEBUG > > > > > /* > > > > > * We should never call set_task_cpu() on a blocked task, > > > > > @@ -1289,15 +1291,14 @@ void set_task_cpu(struct task_struct *p, > > > > > unsigned int new_cpu) > > > > > #endif > > > > > > > > > > trace_sched_migrate_task(p, new_cpu); > > > > > + __set_task_cpu(p, new_cpu); > > > > > > > > > > - if (task_cpu(p) != new_cpu) { > > > > > + if (prev_cpu != new_cpu) { > > > > > if (p->sched_class->migrate_task_rq) > > > > > - p->sched_class->migrate_task_rq(p, new_cpu); > > > > > + p->sched_class->migrate_task_rq(p, prev_cpu); > > > > > p->se.nr_migrations++; > > > > > perf_event_task_migrate(p); > > > > > } > > > > > - > > > > > - __set_task_cpu(p, new_cpu); > > > > > } > > > > > > > > I don't think this is safe, see the comment in __set_task_cpu(). We want > > > > that to be last. > > > > > > I am sorry but I don't understand what you said. I checked the comment in > > > __set_task_cpu(). > > > > > > /* > > > * After ->cpu is set up to a new value, task_rq_lock(p, ...) can be > > > * successfuly executed on another CPU. We must ensure that updates of > > > * per-task data have been completed by this moment. > > > */ > > > > > > Of course, ->cpu should be set up to a new value for task_rq_lock() to be > > > executed successfully on another CPU. Is this the case? Is there something > > > i missed? I think it would be ok if task->pi_lock can work correctly > > > within > > > "if" statement in set_task_cpu(). Is there problem to do that? > > > > So the problem is that as soon as that ->cpu store comes through, the > > other rq->lock can happen, even though we might still hold a rq->lock > > thinking we're serialized. > > > > Take for instance move_queued_tasks(), it does: > > > > dequeue_task(rq, p, 0); > > p->on_rq = TASK_ON_RQ_MIGRATING; > > set_task_cpu(p, new_cpu) { > > __set_task_cpu(); > > > > ^^^ here holding rq->lock is insufficient and the below: > > > > p->sched_class->migrate_task_rq() > > Thank you for explaning in detail, but this's why i asked you. > Yes, rq->lock is insufficient in this place as you said, but > should migrate_task_rq() be serialized by rq->lock? I might have > agreed with you if the migrate_task_rq() should be serialized by > rq->lock, but I think it's not the case. I think it would be of
rq->lock, but I think it's not the case. I think it would be *ok* (sorry for typo) > if task->pi_lock can work correcly within *if statement* in > set_task_cpu(). Wrong? > > > > > would no longer be serialized by rq->lock. > > > > } > > raw_spin_unlock(&rq->lock); > > > > > > -- > > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > > the body of a message to [email protected] > > More majordomo info at http://vger.kernel.org/majordomo-info.html > > Please read the FAQ at http://www.tux.org/lkml/ > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to [email protected] > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
