linux-kernel - Re: [RFC] llist: Fix code comments about llist_del

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Date:   Thu, 8 Dec 2016 18:22:18 -0800
From:   Joel Fernandes <joelaf@...gle.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Will Deacon <will.deacon@....com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC] llist: Fix code comments about llist_del_first locking

On Thu, Dec 8, 2016 at 6:12 PM, Huang, Ying <ying.huang@...el.com> wrote:
> Joel Fernandes <joelaf@...gle.com> writes:
>
>> On Thu, Dec 8, 2016 at 4:42 PM, Joel Fernandes <joelaf@...gle.com> wrote:
>>> On Thu, Dec 8, 2016 at 4:35 PM, Huang, Ying <ying.huang@...el.com> wrote:
>>>> Joel Fernandes <joelaf@...gle.com> 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@...el.com>
>>>>> Cc: Ingo Molnar <mingo@...nel.org>
>>>>> Cc: Will Deacon <will.deacon@....com>
>>>>> Cc: Paul McKenney <paulmck@...ux.vnet.ibm.com>
>>>>> Signed-off-by: Joel Fernandes <joelaf@...gle.com>
>>>>> ---
>>>>>  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".
>
> I tried to describe both cases in the original table.
>
>   *           |   add    | del_first |  del_all
>   * add       |    -     |     -     |     -
>   * del_first |          |     L     |     L
>   * del_all   |          |           |     -
>
> The 'L' for "del_first * del_first" means multiple consumers uses
> llist_del_first() need lock.  And the 'L' for 'del_first * del_all'
> means multiple consumers uses llist_del_first() and llist_del_all() need
> lock.

Ok, now I get it - so basically the table describes one
producer/consumer vs another producer/consumer, in other words you are
just describing contention between any 2 operations. Thanks for
clarifying. I will respin the comments to explain this a bit better if
that's Ok with you.

Regards,
Joel