(cc'ing Frederic) On Wed, Dec 21, 2011 at 09:26:32AM -0800, Tejun Heo wrote: > Hello, guys. > > On Wed, Dec 21, 2011 at 03:44:14AM -0800, tip-bot for Daisuke Nishimura wrote: > > sched: Fix cgroup movement of forking process > > > > There is a small race between task_fork_fair() and sched_move_task(), > > which is trying to move the parent. > > > > task_fork_fair() sched_move_task() > > --------------------------------+--------------------------------- > > cfs_rq = task_cfs_rq(current) > > -> cfs_rq is the "old" one. > > curr = cfs_rq->curr > > -> curr is set to the parent. > > task_rq_lock() > > dequeue_task() > > ->parent.se.vruntime -= (old)cfs_rq->min_vruntime > > enqueue_task() > > ->parent.se.vruntime += (new)cfs_rq->min_vruntime > > task_rq_unlock() > > raw_spin_lock_irqsave(rq->lock) > > se->vruntime = curr->vruntime > > -> vruntime of the child is set to that of the parent > > which has already been updated by sched_move_task(). > > se->vruntime -= (old)cfs_rq->min_vruntime. > > raw_spin_unlock_irqrestore(rq->lock) > > > > As a result, vruntime of the child becomes far bigger than expected, > > if (new)cfs_rq->min_vruntime >> (old)cfs_rq->min_vruntime. > > > > This patch fixes this problem by setting "cfs_rq" and "curr" after > > holding the rq->lock. > > The race shouldn't happen with threadgroup locking scheduled to be > merged for the coming merge window. sched_fork() and cgroup migration > become exclusive and won't happen concurrently. Would still make > sense for -stable tho. I retract that. sched_move_task() can also be called from cgroup_exit() which is outside of threadgroup locking. Frederic, so, it seems we actually have race conditions here. I really wish cgroup made sure that things like this can't happen even if we pay a bit of overhead in relatively cold paths. I could be being unrealistic tho. Any ideas? Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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