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. Best Regards, Huang, Ying > * del_first | | L | L > * del_all | | | - > *
