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SubjectRe: [PATCH v2] llist: Clarify comments about when locking is needed
Date
Joel Fernandes <joelaf@google.com> writes:

> 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.
>
> Cc: Huang Ying <ying.huang@intel.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
> Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Signed-off-by: Joel Fernandes <joelaf@google.com>

Acked-by: "Huang, Ying" <ying.huang@intel.com>

Best Regards,
Huang, Ying

> ---
> 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

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