Re: [RFC] llist: Fix code comments about llist_del_first locking

[Huang\, Ying]

Joel Fernandes <joelaf@xxxxxxxxxx> 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 <ying.huang@xxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> Cc: Paul McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Joel Fernandes <joelaf@xxxxxxxxxx>
> ---
>  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   |          |           |     -
>   *