Messages in this thread | | | Date | Tue, 22 Jun 2010 23:23:57 +0200 | From | Oleg Nesterov <> | Subject | Re: while_each_thread() under rcu_read_lock() is broken? |
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On 06/21, Paul E. McKenney wrote: > > Indeed, the tough part is figuring out when you are done given that things > can come and go at will. Some additional tricks, in no particular order: > > 1. Always start at the group leader.
We can't. We have users which start at the arbitrary thread.
> 2. Maintain a separate task structure that flags the head of the > list. This separate structure is freed one RCU grace period > following the disappearance of the current group leader.
Even simpler, we can just add list_head into signal_struct. I thought about this, but this breaks thread_group_empty (this is fixeable) and, again, I'd like very much to avoid adding new fields into task_struct or signal_struct.
> > Well, another field in task_struct... > > Yeah, would be good to avoid this. Not sure it can be avoided, though.
Why? I think next_thread_careful() from http://marc.info/?l=linux-kernel&m=127714242731448 should work.
If the caller holds tasklist or siglock, this change has no effect.
If the caller does while_each_thread() under rcu_read_lock(), then it is OK to break the loop earlier than we do now. The lockless while_each_thread() works in a "best effort" manner anyway, if it races with exit_group() or exec() it can miss some/most/all sub-threads (including the new leader) with or without this change.
Yes, zap_threads() needs additional fixes. But I think it is better to complicate a couple of lockless callers (or just change them to take tasklist) which must not miss an "interesting" thread.
> > > o Do the de_thread() incrementally. So if the list is tasks A, > > > B, and C, in that order, and if we are de-thread()ing B, > > > then make A's pointer refer to C, > > > > This breaks while_each_thread() under tasklist/siglock. It must > > see all unhashed tasks. > > Could de_thread() hold those locks in order to avoid that breakage?
How can it hold, say, siglock? We need to wait a grace period. To clarify. de_thread() kills all threads except the group_leader, so we have only 2 threads: group_leader A and B.
If we add synchronize_rcu() before release_task(leader) (as Roland suggested), then we don't need to change A's pointer. This probably fixes while_each_thread() in the common case. But this disallows the tricks like rcu_lock_break().
And. Whatever we do with de_thread(), this can't fix the lockless while_each_thread(not_a_group_leader, t). I do not know if there is any user which does this though. fastpath_timer_check()->thread_group_cputimer() does this, but this is wrong and we already have the patch which removes it.
Oleg.
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