Hi Paul, On Mon, May 4, 2020 at 3:01 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > On Mon, May 04, 2020 at 02:08:05PM -0400, Joel Fernandes wrote: > > On Mon, May 04, 2020 at 07:48:22PM +0200, Uladzislau Rezki wrote: > > > On Mon, May 04, 2020 at 08:24:37AM -0700, Paul E. McKenney wrote: > > [..] > > > > > > Presumably the list can also be accessed without holding this lock, > > > > > > because otherwise we shouldn't need llist... > > > > > > > > > > > Hm... We increase the number of elements in cache, therefore it is not > > > > > lockless. From the other hand i used llist_head to maintain the cache > > > > > because it is single linked list, we do not need "*prev" link. Also > > > > > we do not need to init the list. > > > > > > > > > > But i can change it to list_head. Please let me know if i need :) > > > > > > > > Hmmm... Maybe it is time for a non-atomic singly linked list? In the RCU > > > > callback processing, the operations were open-coded, but they have been > > > > pushed into include/linux/rcu_segcblist.h and kernel/rcu/rcu_segcblist.*. > > > > > > > > Maybe some non-atomic/protected/whatever macros in the llist.h file? > > > > Or maybe just open-code the singly linked list? (Probably not the > > > > best choice, though.) Add comments stating that the atomic properties > > > > of the llist functions aren't neded? Something else? > > > > > > > In order to keep it simple i can replace llist_head by the list_head? > > > > Just to clarify for me, what is the disadvantage of using llist here? > > Are there some llist APIs that are not set up for concurrency? I am > not seeing any. llist deletion racing with another llist deletion will need locking. So strictly speaking, some locking is possible with llist usage? The locklessness as I understand comes when adding and deleting at the same time. For that no lock is needed. But in the current patch, it locks anyway to avoid the lost-update of the size of the list. > The overhead isn't that much of a concern, given that these are not on the > hotpath, but people reading the code and seeing the cmpxchg operations > might be forgiven for believing that there is some concurrency involved > somewhere. > > Or am I confused and there are now single-threaded add/delete operations > for llist? I do see some examples of llist usage with locking in the kernel code. One case is: do_init_module() calling llist_add to add to the init_free_list under module_mutex. > > Since we don't care about traversing backwards, isn't it better to use llist > > for this usecase? > > > > I think Vlad is using locking as we're also tracking the size of the llist to > > know when to free pages. This tracking could suffer from the lost-update > > problem without any locking, 2 lockless llist_add happened simulatenously. > > > > Also if list_head is used, it will take more space and still use locking. > > Indeed, it would be best to use a non-concurrent singly linked list. Ok cool :-) Is it safe to say something like the following is ruled out? ;-) :-D #define kfree_rcu_list_add llist_add Thanks, - Joel
