On Mon, Nov 16, 2015 at 01:53:51PM +0100, Peter Zijlstra wrote: > 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: > > > 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 > > if task->pi_lock can work correcly within *if statement* in > > set_task_cpu(). Wrong? > > So currently, set_task_cpu() is serialized by: > > - p->pi_lock; on wakeup > - rq->lock; otherwise > > (see the #ifdef CONFIG_LOCKDEP comment in set_task_cpu())
I already read the comment.. Then do you mean the comment above migrate_task_rq_fair() is wrong and should be fixed? I thought the comment above migrate_task_rq_fair() is correct rather than CONFIG_LOCKDEP comment in set_task_cpu(), when I read it. I think these two comments are conflict each other a little bit, so one of those should be fixed. * the comment above migrate_task_rq_fair() describes it like, Caller SHOULD HOLD (&p->pi_lock) * the CONFIG_LOCKDEP comment in set_task_cpu() describes it like, Caller SHOULD HOLD (&p->pi_lock || &rq->lock) > > This means that sched_class::migrate_task() cannot indeed rely on > rq->lock for full serialization, however it still means that > task_rq_lock() will fully serialize against the thing. Yes I also think this is true. > > By changing this, it no longer will. ??? > > Even without that; I think such a change, if correct, is very fragile > and prone to creating problems later on, and sets bad precedent. > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majord...@vger.kernel.org > 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 majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/