Re: [PATCH v2] llist: Clarify comments about when locking is needed
From: Joel Fernandes
Date: Sat Dec 10 2016 - 13:20:48 EST
On Sat, Dec 10, 2016 at 10:15 AM, Mathieu Desnoyers
> ----- On Dec 10, 2016, at 7:03 PM, Joel Fernandes joelaf@xxxxxxxxxx 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.
>> Cc: Huang Ying <ying.huang@xxxxxxxxx>
>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>> Cc: Will Deacon <will.deacon@xxxxxxx>
>> Cc: Paul McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
>> Acked-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
>> Signed-off-by: Joel Fernandes <joelaf@xxxxxxxxxx>
>> v2 changes:
>> Minor changes to comment and commit message based on Mathieu's suggestions
>> 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
> Mathieu Desnoyers
> EfficiOS Inc.