llist: Clarify comments about when locking is needed

llist.h comments are a bit confusing about when locking is needed versus when it isn't. Clarify these comments a bit more and be a bit more descriptive about why locking is needed for llist_del_first.

Cc Huang Ying <[email protected]>

Cc Ingo Molnar <[email protected]>

Cc Will Deacon <[email protected]>

Cc Paul McKenney <[email protected]>

Cc Mathieu Desnoyers <[email protected]>

Signed-off-by Joel Fernandes <[email protected]>

---
include/linux/llist.h | 37 +++++++++++++++++++++----------------

1 file changed, 21 insertions(+), 16 deletions(-)

include/linux/llist.h +4 -14

index fd4ca0b..31822bb 100644
@@ -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