Messages in this thread | | | Date | Tue, 25 Aug 2015 12:10:32 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] sched: fix tsk->pi_lock isn't held when do_set_cpus_allowed() |
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On Tue, Aug 25, 2015 at 12:05:27PM +0200, Peter Zijlstra wrote: > On Tue, Aug 25, 2015 at 03:59:54PM +0800, Wanpeng Li wrote: > > +++ b/kernel/cpuset.c > > @@ -2376,8 +2376,12 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask) > > > > void cpuset_cpus_allowed_fallback(struct task_struct *tsk) > > { > > + unsigned long flags; > > + > > rcu_read_lock(); > > + raw_spin_lock_irqsave(&tsk->pi_lock, flags); > > do_set_cpus_allowed(tsk, task_cs(tsk)->effective_cpus); > > + raw_spin_lock_irqsave(&tsk->pi_lock, flags); > > rcu_read_unlock(); > > Aside from the double lock thing that was already pointed out, I think > this is wrong, because the select_task_rq() call can already have > pi_lock held. > > Taking it again would result in a deadlock. > > Consider for instance: > > try_to_wake_up() > raw_spin_lock_irqsave(->pi_lock) > select_task_rq() > select_ballback_rq() > cpuset_cpus_allowed_fallback() > raw_spin_lock_irqsave(->pi_lock) > > > The problem is with the migration path and should be fixed there.
Another problem, migration_call() will have rq->lock held, so you're proposing to acquire pi_lock while holding rq->lock, this is an inversion from the regular nesting order.
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