Messages in this thread | | | Date | Mon, 21 Jun 2010 22:06:33 +0200 | From | Oleg Nesterov <> | Subject | Re: while_each_thread() under rcu_read_lock() is broken? |
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On 06/21, Roland McGrath wrote: > > > Paul, Roland, do you see any problems from the correctness pov, > > or a better fix for now? > > > > Perhaps it also makes sense to keep the old variant renamed to > > while_each_thread_locked(), I dunno. > > Did we verify that only de_thread() can create the situation where a > while_each_thread-style loop without either lock can be confused?
I think yes, this is is the only case.
I mean, while_each_thread(group_leader, t). If g != group_leader, then the lockless while_each_thread() has problems with the plain exit(g).
Afaics. The more I think about this, the more I feel confused ;)
But if we start from ->group_leader, then while_each_thread() must stop eventually. Otherwise we should assume that the dead (unhashed) tasks can create the circular list, obviously this is not possible.
> If > that's so, then just changing it to avoid the situation seems like it > would be less invasive overall.
How? We should change ->group_leader uner write_lock_irq(tasklist), synchronize_rcu() is not an option. We can't do call_rcu(release_task), we can't take tasklist for writing in the softirq context. But even if we could, this can't help in fact or I missed something.
Oleg.
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