Messages in this thread | | | Date | Thu, 31 Jul 2014 19:27:52 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH v2 tip/core/rcu 01/10] rcu: Add call_rcu_tasks() |
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On 07/31, Paul E. McKenney wrote: > > On Thu, Jul 31, 2014 at 06:31:38PM +0200, Oleg Nesterov wrote: > > > But can't we avoid get_task_struct()? This can pin a lot of task_struct's. > > Can't we just add list_del_rcu(holdout_list) into __unhash_process() ? > > If I add the list_del_rcu() there, then I am back to a concurrent list, > which I would like to avoid. Don't get me wrong, it was fun playing with > the list-locked stuff, but best to avoid it if we can.
OK,
> The nice thing about using get_task_struct to lock > them down is that -only- the task_struct itself is locked down -- the > task can be reaped and so on.
I understand. but otoh it would be nice to not pin this memory if the task was already (auto)reaped.
And afaics the number of pinned task_struct's is not bounded. In theory it is not even limited by, say, PID_MAX_LIMIT. A thread can exit and reap itself right after get_task_struct() but create another running thread which can be noticed by rcu_tasks_kthread() too.
> > We only need to ensure that list_add() above can't race with that list_del(), > > perhaps we can tolerate lock_task_sighand() ? > > I am worried about a task that does a voluntary context switch, then exits. > This could results in rcu_tasks_kthread() and __unhash_process() both > wanting to dequeue at the same time, right?
Oh yes, I was very wrong. And we do not want to abuse tasklist_lock...
OK, let me try to read the patch first.
Oleg.
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