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. 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? > 5. Just mark the deleted elements, but leave them in the list. > Actually remove them using one of the above techniques. Honestly, I have a plan to do this thing as a future work. But now, I can assume deletion never happen with the list :) > I suggest that such special cases stay in the subsystem in question. > If a given technique gains wider use, then it might be time to > update header comments. Ok. > > If yes, we may add another function traversing starting from a head. Or > > just use existing funtion with head->first. > > If you start with head->first, then you need to make sure that a concurrent > add of an element at the head of the list works. You need at least a > READ_ONCE() and preferably an rcu_dereference() or similar. Yes, sir. I'll be careful in doing it. Thanks a lot. > > Thank a lot for your answers in advance :) > > You did ask! > > Thanx, Paul > > > ----->8----- > > >From 1e73882799b269cd86e7a7c955021e3a18d1e6cf Mon Sep 17 00:00:00 2001 > > From: Byungchul Park <byungchul.p...@lge.com> > > Date: Tue, 31 Jul 2018 09:31:57 +0900 > > Subject: [QUESTION] llist: Comment releasing 'must delete' restriction > > before > > traversing > > > > llist traversing can run without deletion in restrictive cases all > > items are added but never deleted like a rculist traversing such as > > list_for_each_entry_lockless. So add the comment. > > > > Signed-off-by: Byungchul Park <byungchul.p...@lge.com> > > --- > > include/linux/llist.h | 24 ++++++++++++++++++------ > > 1 file changed, 18 insertions(+), 6 deletions(-) > > > > diff --git a/include/linux/llist.h b/include/linux/llist.h > > index 85abc29..d012d3e 100644 > > --- a/include/linux/llist.h > > +++ b/include/linux/llist.h > > @@ -32,8 +32,12 @@ > > * operation, with "-" being no lock needed, while "L" being lock is > > needed. > > * > > * The list entries deleted via llist_del_all can be traversed with > > - * traversing function such as llist_for_each etc. But the list > > - * entries can not be traversed safely before deleted from the list. > > + * traversing function such as llist_for_each etc. Normally the list > > + * entries cannot be traversed safely before deleted from the list > > + * except the cases items are added to the list but never deleted. In > > + * that restrictive cases the list may be safely traversed concurrently > > + * with llist_add. > > + * > > * The order of deleted entries is from the newest to the oldest added > > * one. If you want to traverse from the oldest to the newest, you > > * must reverse the order by yourself before traversing. > > @@ -116,7 +120,9 @@ static inline void init_llist_head(struct llist_head > > *list) > > * > > * In general, some entries of the lock-less list can be traversed > > * safely only after being deleted from list, so start with an entry > > - * instead of list head. > > + * instead of list head. But in restrictive cases items are added to > > + * the list but never deleted, the list may be safely traversed > > + * concurrently with llist_add. > > * > > * If being used on entries deleted from lock-less list directly, the > > * traverse order is from the newest to the oldest added entry. If > > @@ -135,7 +141,9 @@ static inline void init_llist_head(struct llist_head > > *list) > > * > > * In general, some entries of the lock-less list can be traversed > > * safely only after being deleted from list, so start with an entry > > - * instead of list head. > > + * instead of list head. But in restrictive cases items are added to > > + * the list but never deleted, the list may be safely traversed > > + * concurrently with llist_add. > > * > > * If being used on entries deleted from lock-less list directly, the > > * traverse order is from the newest to the oldest added entry. If > > @@ -153,7 +161,9 @@ static inline void init_llist_head(struct llist_head > > *list) > > * > > * In general, some entries of the lock-less list can be traversed > > * safely only after being removed from list, so start with an entry > > - * instead of list head. > > + * instead of list head. But in restrictive cases items are added to > > + * the list but never deleted, the list may be safely traversed > > + * concurrently with llist_add. > > * > > * If being used on entries deleted from lock-less list directly, the > > * traverse order is from the newest to the oldest added entry. If > > @@ -175,7 +185,9 @@ static inline void init_llist_head(struct llist_head > > *list) > > * > > * In general, some entries of the lock-less list can be traversed > > * safely only after being removed from list, so start with an entry > > - * instead of list head. > > + * instead of list head. But in restrictive cases items are added to > > + * the list but never deleted, the list may be safely traversed > > + * concurrently with llist_add. > > * > > * If being used on entries deleted from lock-less list directly, the > > * traverse order is from the newest to the oldest added entry. If > > -- > > 1.9.1 > >
