Hi, Pual, "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> writes: > On Mon, Dec 11, 2017 at 01:30:03PM +0800, Huang, Ying wrote: >> Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes: >> >> > On Fri, 08 Dec 2017 16:41:38 +0800 "Huang\, Ying" <ying.huang@xxxxxxxxx> wrote: >> > >> >> > Why do we need srcu here? Is it enough with rcu like below? >> >> > >> >> > It might have a bug/room to be optimized about performance/naming. >> >> > I just wanted to show my intention. >> >> >> >> Yes. rcu should work too. But if we use rcu, it may need to be called >> >> several times to make sure the swap device under us doesn't go away, for >> >> example, when checking si->max in __swp_swapcount() and >> >> add_swap_count_continuation(). And I found we need rcu to protect swap >> >> cache radix tree array too. So I think it may be better to use one >> >> calling to srcu_read_lock/unlock() instead of multiple callings to >> >> rcu_read_lock/unlock(). >> > >> > Or use stop_machine() ;) It's very crude but it sure is simple. Does >> > anyone have a swapoff-intensive workload? >> >> Sorry, I don't know how to solve the problem with stop_machine(). >> >> The problem we try to resolved is that, we have a swap entry, but that >> swap entry can become invalid because of swappoff between we check it >> and we use it. So we need to prevent swapoff to be run between checking >> and using. >> >> I don't know how to use stop_machine() in swapoff to wait for all users >> of swap entry to finish. Anyone can help me on this? > > You can think of stop_machine() as being sort of like a reader-writer > lock. The readers can be any section of code with preemption disabled, > and the writer is the function passed to stop_machine(). > > Users running real-time applications on Linux don't tend to like > stop_machine() much, but perhaps it is nevertheless the right tool > for this particular job. Thanks a lot for explanation! Now I understand this. Another question, for this specific problem, I think both stop_machine() based solution and rcu_read_lock/unlock() + synchronize_rcu() based solution work. If so, what is the difference between them? I guess rcu based solution will be a little better for real-time applications? So what is the advantage of stop_machine() based solution? Best Regards, Huang, Ying -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
