Again, sorry, I didn't read the patches yet, just noticed your discussion...
On 07/31, Paul E. McKenney wrote: > > On Thu, Jul 31, 2014 at 03:30:12PM +0800, Lai Jiangshan wrote: > > > > + t->rcu_tasks_nvcsw = ACCESS_ONCE(t->nvcsw); > > > + t->rcu_tasks_holdout = 1; > > > + list_add(&t->rcu_tasks_holdout_list, > > > + &rcu_tasks_holdouts); > > > > I think get_task_struct() is needed here to avoid the task disappears. > > Hmmm... Let's see... > > Looks like get_task_struct() does a blind atomic increment of ->usage. > And put_task_struct() does an atomic_dec_and_test(). So one question > is "what prevents us from doing get_task_struct() after the final > put_task_struct() has pushed ->usage down to zero?" > > Hopefully there is a grace period in there somewhere, otherwise it will > be necessary to take the task-list lock, which I would like to avoid. > > Looks like the call_rcu() of delayed_put_task_struct() in release_task() > might be doing this. Yes, exactly, so get_task_struct() is always fine as long as task_struct itself is protected by RCU. 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() ? We only need to ensure that list_add() above can't race with that list_del(), perhaps we can tolerate lock_task_sighand() ? Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
