Messages in this thread | | | Date | Sat, 18 Oct 2014 22:56:14 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH] sched/numa: fix unsafe get_task_struct() in task_numa_assign() |
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On 10/18, Kirill Tkhai wrote: > > 18.10.2014, 01:40, "Oleg Nesterov" <oleg@redhat.com>: > > ... > > The > > task_struct itself can't go away, > > ... > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -1158,7 +1158,13 @@ static void task_numa_compare(struct task_numa_env *env, > > > > rcu_read_lock(); > > cur = ACCESS_ONCE(dst_rq->curr); > > - if (cur->pid == 0) /* idle */ > > + /* > > + * No need to move the exiting task, and this ensures that ->curr > > + * wasn't reaped and thus get_task_struct() in task_numa_assign() > > + * is safe; note that rcu_read_lock() can't protect from the final > > + * put_task_struct() after the last schedule(). > > + */ > > + if (is_idle_task(cur) || (cur->flags & PF_EXITING)) > > cur = NULL; > > > > /* > > Oleg, I've looked once again, and now it's not good for me.
Ah. Thanks a lot Kirill for correcting me!
I was looking at this rcu_read_lock() and I didn't even try to think what it can actually protect. Nothing.
> --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -1165,7 +1165,21 @@ static void task_numa_compare(struct task_numa_env *env, >^^ > rcu_read_lock(); > cur = ACCESS_ONCE(dst_rq->curr); > - if (cur->pid == 0) /* idle */ > + /* > + * No need to move the exiting task, and this ensures that ->curr > + * wasn't reaped and thus get_task_struct() in task_numa_assign() > + * is safe; note that rcu_read_lock() can't protect from the final > + * put_task_struct() after the last schedule(). > + */ > + if (is_idle_task(cur) || (cur->flags & PF_EXITING)) > + cur = NULL; > + /* > + * Check once again to be sure curr is still on dst_rq. Even if > + * it points on a new task, which is using the memory of freed > + * cur, it's OK, because we've locked RCU before > + * delayed_put_task_struct() callback is called to put its struct. > + */ > + if (cur != ACCESS_ONCE(dst_rq->curr))
No, I don't think this can work. Let's look at the current code:
rcu_read_lock(); cur = ACCESS_ONCE(dst_rq->curr); if (cur->pid == 0) /* idle */
And any dereference, even reading ->pid is not safe. This memory can be freed, unmapped, reused, etc.
Looks like, task_numa_compare() needs to take dst_rq->lock and get the refernce first.
Or, perhaps, we need to change the rules to ensure that any "task_struct *" pointer is rcu-safe. Perhaps we have more similar problems... I'd like to avoid this if possible.
Hmm. I'll try to think more.
Thanks!
Oleg.
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