On Tue, Jul 31, 2018 at 07:30:52AM -0700, Paul E. McKenney wrote: > On Tue, Jul 31, 2018 at 06:29:50PM +0900, Byungchul Park wrote: > > On Mon, Jul 30, 2018 at 09:30:42PM -0700, Paul E. McKenney wrote: > > > On Tue, Jul 31, 2018 at 09:58:36AM +0900, Byungchul Park wrote: > > > > Hello folks, > > > > > > > > I'm careful in saying.. and curious about.. > > > > > > > > In restrictive cases like only addtions happen but never deletion, can't > > > > we safely traverse a llist? I believe llist can be more useful if we can > > > > release the restriction. Can't we? > > > > > > Yes, but please give a thought to the people looking at your code some > > > time down the line. If you are doing this, lots of comments, please. > > > > Yes, I will. Thank you for the comment. > > > > > Here are the approaches that I am aware of: > > > > > > 1. Normal RCU. Use list_add_rcu(), list_del_rcu(), and friends. > > > > > > 2. Things are added but never deleted. Use list_add_rcu() and > > > friends, but since you don't ever delete anything, you never > > > use list_del_rcu(), synchronize_rcu(), call_rcu(), and friends. > > > > I think rcu list also works well. But I decided to use llist because > > llist is simpler and has one less pointer. > > No. > > To see this, look at llist_for_each() below, which is absolutely -not- > able to reliably traverse lists while nodes are being inserted.
Ah.. Not only 'node' but also 'pos->next' can cause a problem w/o rcu_dereference or similar.. I need consider another way. Or I might need memory barrier between READ_ONCE(head->first) and llist_for_each() to make sure safely to read 'pos->next' when I see a value of 'head->first'. > #define llist_for_each(pos, node) \ > for ((pos) = (node); pos; (pos) = (pos)->next) > > Now, you could introduce an llist_for_each_rcu() that used rcu_dereference > or similar (thus handling insertion, but that is not what your patches > currently do. Right. > > Just to be sure, let me explain my use case more: > > > > 1. Introduced a global list where single linked list is sufficient. > > 2. All operations I need is to add items and traverse the list. > > 3. Ensure the operations always happen within irq-disabled section. > > 4. I'm considering CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG properly. > > 5. The list can be accessed by every CPU concurrently. > > > > Of course, I can use normal double list with a lock or rcu list. But I > > think it doesn't have to be protected by even rcu in that case. I wanted > > to use the simplest one all requiremnets are satisfied with and I > > thought it's llist. Thoughts? > > If you want lockless reader traversal, you need rcu_dereference(). Yes, I need that or similar anyway. Thanks a lot, Paul!
