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

[Joel Fernandes]

On Thu, Dec 8, 2016 at 4:42 PM, Joel Fernandes <joelaf@xxxxxxxxxx> wrote:
> On Thu, Dec 8, 2016 at 4:35 PM, Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>> 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.
>
> 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