On Wed, Nov 18, 2015 at 12:55:10AM +0100, Peter Zijlstra wrote: > On Wed, Nov 18, 2015 at 08:37:00AM +0900, Byungchul Park wrote: > > > Which one do you think to be fixed? The one above migrate_task_rq_fair()? > > I wonder if it would be ok even it does not hold pi_lock in > > migrate_task_rq_fair(). If you say *no problem*, I will try to fix the > > comment. > > The one above migrate_task_rq_fair() is obviously broken, as > demonstrated by the move_queued_task() case. > > Also, pretty much all runnable task migration code will not take > pi_lock, see also {pull,push}_{rt,dl}_task(). > > Note that this is done very much by design, task_rq_lock() is the thing > that fully serializes a task's scheduler state. Runnable tasks use > rq->lock, waking tasks use pi_lock. > > > > I meant, if you call __set_task_cpu() before > > > sched_class::migrate_task_rq(), in that case task_rq_lock() will no > > > longer fully serialize against set_task_cpu(). > > > > > > Because once you've called __set_task_cpu(), task_rq_lock() will acquire > > > the _other_ rq->lock. And we cannot rely on our rq->lock to serialize > > > things. > > > > I agree with you if migtrate_task_rq() can be serialized by rq->lock > > without holding pi_lock. (even though I am still wondering..) > > move_queued_task() illustrates this. > > > But I thought it was no problem if migrate_task_rq() was serialized only > > by pi_lock as the comment above the migrate_task_rq() describes, because > > breaking rq->lock does not affect the sericalization by pi_lock. > > Right, but per the above, we cannot assume pi_lock is in fact held over > this.
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