linux-kernel - Re: [PATCH v2] llist: Clarify comments about when locking is needed

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Message-ID: <CAJWu+oriHJ3i0pXVa+MHKHs7KB6SBqBHvfeT4+FPN0-5+-dy2A@mail.gmail.com>
Date:   Sat, 10 Dec 2016 10:20:42 -0800
From:   Joel Fernandes <joelaf@...gle.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Huang Ying <ying.huang@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        Will Deacon <will.deacon@....com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2] llist: Clarify comments about when locking is needed

On Sat, Dec 10, 2016 at 10:15 AM, Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
> ----- On Dec 10, 2016, at 7:03 PM, Joel Fernandes joelaf@...gle.com wrote:
>
>> llist.h comments are a bit confusing about when locking is needed versus when
>> it isn't. Clarify these comments a bit more by being a bit more descriptive
>> about why locking is needed for llist_del_first.
>
> As I stated in my earlier review, please remove a couple of "a bit"
> from the changelog.
>

I'm sorry I missed that. I will update it correctly in the next rev.

Regards,
Joel



> Thanks,
>
> Mathieu
>
>>
>> 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>
>> Acked-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
>> Signed-off-by: Joel Fernandes <joelaf@...gle.com>
>> ---
>> v2 changes:
>> Minor changes to comment and commit message based on Mathieu's suggestions
>> (https://lkml.org/lkml/2016/12/10/39)
>>
>> include/linux/llist.h | 37 +++++++++++++++++++++----------------
>> 1 file changed, 21 insertions(+), 16 deletions(-)
>>
>> diff --git a/include/linux/llist.h b/include/linux/llist.h
>> index fd4ca0b..31822bb 100644
>> --- a/include/linux/llist.h
>> +++ b/include/linux/llist.h
>> @@ -3,28 +3,33 @@
>> /*
>>  * 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 one consumer, llist_add can be
>> - * used in producers and llist_del_all or llist_del_first can be used
>> - * in the consumer.
>> - *
>> - * This can be summarized as follow:
>> + * Cases where locking is not needed:
>> + * If there are multiple producers and multiple consumers, llist_add can be
>> + * used in producers and llist_del_all can be used in consumers simultaneously
>> + * without locking. Also a single consumer can use llist_del_first while
>> + * multiple producers simultaneously use llist_add, without any locking.
>> + *
>> + * Cases where locking is needed:
>> + * If we have multiple consumers with llist_del_first used in one consumer, and
>> + * llist_del_first or llist_del_all used in other consumers, then a lock is
>> + * needed.  This is because llist_del_first depends on list->first->next not
>> + * changing, but without lock protection, there's no way to be sure about that
>> + * if a preemption happens in the middle of the delete operation and on being
>> + * preempted back, the list->first is the same as before causing the cmpxchg in
>> + * llist_del_first to succeed. For example, while a llist_del_first operation
>> + * is in progress in one consumer, then a llist_del_first, llist_add,
>> + * llist_add (or llist_del_all, llist_add, llist_add) sequence in another
>> + * consumer may cause violations.
>> + *
>> + * This can be summarized as follows:
>>  *
>>  *           |   add    | del_first |  del_all
>>  * add       |    -     |     -     |     -
>>  * del_first |          |     L     |     L
>>  * del_all   |          |           |     -
>>  *
>> - * Where "-" stands for no lock is needed, while "L" stands for lock
>> - * is needed.
>> + * Where, a particular row's operation can happen concurrently with a column's
>> + * operation, with "-" being no lock needed, while "L" being lock is needed.
>>  *
>>  * The list entries deleted via llist_del_all can be traversed with
>>  * traversing function such as llist_for_each etc.  But the list
>> --
>> 2.8.0.rc3.226.g39d4020
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com