On Thu, Dec 8, 2016 at 4:42 PM, Joel Fernandes <[email protected]> wrote: > On Thu, Dec 8, 2016 at 4:35 PM, Huang, Ying <[email protected]> wrote: >> Joel Fernandes <[email protected]> writes: >> >>> Usage llist_del_first needs lock protection, however the table in the >>> comments of llist.h show a '-'. Correct this, and also add better >>> comments on top. >>> >>> Cc: Huang Ying <[email protected]> >>> Cc: Ingo Molnar <[email protected]> >>> Cc: Will Deacon <[email protected]> >>> Cc: Paul McKenney <[email protected]> >>> Signed-off-by: Joel Fernandes <[email protected]> >>> --- >>> include/linux/llist.h | 19 ++++++++++--------- >>> 1 file changed, 10 insertions(+), 9 deletions(-) >>> >>> diff --git a/include/linux/llist.h b/include/linux/llist.h >>> index fd4ca0b..15e4949 100644 >>> --- a/include/linux/llist.h >>> +++ b/include/linux/llist.h >>> @@ -3,14 +3,15 @@ >>> /* >>> * Lock-less NULL terminated single linked list >>> * >>> - * If there are multiple producers and multiple consumers, llist_add >>> - * can be used in producers and llist_del_all can be used in >>> - * consumers. They can work simultaneously without lock. But >>> - * llist_del_first can not be used here. Because llist_del_first >>> - * depends on list->first->next does not changed if list->first is not >>> - * changed during its operation, but llist_del_first, llist_add, >>> - * llist_add (or llist_del_all, llist_add, llist_add) sequence in >>> - * another consumer may violate that. >>> + * If there are multiple producers and multiple consumers, llist_add can be >>> + * used in producers and llist_del_all can be used in consumers. They can >>> work >>> + * simultaneously without lock. But llist_del_first will need to use a >>> lock >>> + * with any other operation (ABA problem). This is because llist_del_first >>> + * depends on list->first->next not changing but there's no way to be sure >>> + * about that and the cmpxchg in llist_del_first may succeed if >>> list->first is >>> + * the same after concurrent operations. For example, a llist_del_first, >>> + * llist_add, llist_add (or llist_del_all, llist_add, llist_add) sequence >>> in >>> + * another consumer may cause violations. >>> * >>> * If there are multiple producers and one consumer, llist_add can be >>> * used in producers and llist_del_all or llist_del_first can be used >>> @@ -19,7 +20,7 @@ >>> * This can be summarized as follow: >>> * >>> * | add | del_first | del_all >>> - * add | - | - | - >>> + * add | - | L | - >> >> If there are only one consumer which only calls llist_del_first(), lock >> is unnecessary. So '-' is shown here originally. But if there are >> multiple consumers which call llist_del_first() or llist_del_all(), lock >> is needed. > > I think this needs to be made more clear in the table. The table > doesn't clear say whether it describes the preceding paragraph > (multiple producers and one consumer), or if it describes the multiple > producers and one consumer case. So either we should have 2 tables, or
Sorry, I meant "or if it describes the multiple producer and multiple consumer case". Regards, Joel
