It sometimes is necessary to be able to be able to use llist in
the following manner:
if (node_unlisted(node))
llst_add(node, list);
i. e. only add a node to the list if it's not already on a list.
This is not possible without taking locks because otherwise there's
an obvious race between the check and the addition. This patch adds
the routine to add a node only if it is not on any list that is
lockless and race free as long as there's only one consumer.
That is, llist_add_exclusive would only add a node if it's not
already on a list, and llist_del_first_exclusive will delete the
first node off the list and mark it as not being on any list.
Signed-off-by: Vitaly Wool <[email protected]>
---
include/linux/llist.h | 25 +++++++++++++++++++++++++
lib/llist.c | 29 +++++++++++++++++++++++++++++
2 files changed, 54 insertions(+)
diff --git a/include/linux/llist.h b/include/linux/llist.h
index 85abc29..5c60e9e 100644
--- a/include/linux/llist.h
+++ b/include/linux/llist.h
@@ -74,6 +74,10 @@ struct llist_node {
#define LLIST_HEAD_INIT(name) { NULL }
#define LLIST_HEAD(name) struct llist_head name = LLIST_HEAD_INIT(name)
+#define LLIST_NODE_UNLISTED ((void *)(-1L))
+#define LLIST_NODE_INIT(name) { LLIST_NODE_UNLISTED }
+#define LLIST_NODE(name) struct llist_node name = LLIST_NODE_INIT(name)
+
/**
* init_llist_head - initialize lock-less list head
* @head: the head for your lock-less list
@@ -238,4 +242,25 @@ extern struct llist_node *llist_del_first(struct llist_head *head);
struct llist_node *llist_reverse_order(struct llist_node *head);
+/**
+ * llist_del_first_exclusive - delete the first entry of lock-less list
+ * and make sure its ->next is NULL
+ * @head: the head for your lock-less list
+ *
+ * If list is empty, return NULL, otherwise, return the first entry
+ * deleted, this is the newest added one.
+ *
+ */
+static inline struct llist_node *llist_del_first_exclusive(
+ struct llist_head *head)
+{
+ struct llist_node *node = llist_del_first(head);
+
+ if (READ_ONCE(node))
+ smp_store_release(&node->next, LLIST_NODE_UNLISTED);
+ return node;
+}
+
+extern bool llist_add_exclusive(struct llist_node *, struct llist_head *);
+
#endif /* LLIST_H */
diff --git a/lib/llist.c b/lib/llist.c
index 7062e93..c85fa6b 100644
--- a/lib/llist.c
+++ b/lib/llist.c
@@ -102,3 +102,32 @@ struct llist_node *llist_reverse_order(struct llist_node *head)
return new_head;
}
EXPORT_SYMBOL_GPL(llist_reverse_order);
+
+/**
+ * llist_add_exclusive - add a node only if its ->next is NULL
+ * @node: the node to be added
+ * @head: the head for your lock-less list
+ *
+ * Return true if the node was added, or false otherwise.
+ */
+bool llist_add_exclusive(struct llist_node *node, struct llist_head *head)
+{
+ struct llist_node *next = LLIST_NODE_UNLISTED;
+ struct llist_node *entry, *new_next;
+
+ /*
+ * if new_next is next (== LLIST_NODE_UNLISTED) on the first run, we
+ * get exclusive access to this node as long as others use
+ * llist_add_safe too.
+ * Then false can be returned no more and we'll loop until we get the
+ * stuff right.
+ */
+ do {
+ entry = READ_ONCE(head->first);
+ if ((new_next = cmpxchg(&node->next, next, entry)) != next)
+ return false;
+ next = entry;
+ } while (cmpxchg(&head->first, entry, node) != entry);
+ return true;
+}
+EXPORT_SYMBOL_GPL(llist_add_exclusive);
--
2.7.4
Vitaly Wool <[email protected]> writes:
> It sometimes is necessary to be able to be able to use llist in
> the following manner:
> if (node_unlisted(node))
> llst_add(node, list);
> i. e. only add a node to the list if it's not already on a list.
>
> This is not possible without taking locks because otherwise there's
> an obvious race between the check and the addition. This patch adds
> the routine to add a node only if it is not on any list that is
> lockless and race free as long as there's only one consumer.
>
> That is, llist_add_exclusive would only add a node if it's not
> already on a list, and llist_del_first_exclusive will delete the
> first node off the list and mark it as not being on any list.
>
> Signed-off-by: Vitaly Wool <[email protected]>
> ---
> include/linux/llist.h | 25 +++++++++++++++++++++++++
> lib/llist.c | 29 +++++++++++++++++++++++++++++
> 2 files changed, 54 insertions(+)
>
> diff --git a/include/linux/llist.h b/include/linux/llist.h
> index 85abc29..5c60e9e 100644
> --- a/include/linux/llist.h
> +++ b/include/linux/llist.h
> @@ -74,6 +74,10 @@ struct llist_node {
> #define LLIST_HEAD_INIT(name) { NULL }
> #define LLIST_HEAD(name) struct llist_head name = LLIST_HEAD_INIT(name)
>
> +#define LLIST_NODE_UNLISTED ((void *)(-1L))
> +#define LLIST_NODE_INIT(name) { LLIST_NODE_UNLISTED }
> +#define LLIST_NODE(name) struct llist_node name = LLIST_NODE_INIT(name)
> +
> /**
> * init_llist_head - initialize lock-less list head
> * @head: the head for your lock-less list
> @@ -238,4 +242,25 @@ extern struct llist_node *llist_del_first(struct llist_head *head);
>
> struct llist_node *llist_reverse_order(struct llist_node *head);
>
> +/**
> + * llist_del_first_exclusive - delete the first entry of lock-less list
> + * and make sure its ->next is NULL
> + * @head: the head for your lock-less list
> + *
> + * If list is empty, return NULL, otherwise, return the first entry
> + * deleted, this is the newest added one.
> + *
> + */
> +static inline struct llist_node *llist_del_first_exclusive(
> + struct llist_head *head)
> +{
> + struct llist_node *node = llist_del_first(head);
> +
> + if (READ_ONCE(node))
> + smp_store_release(&node->next, LLIST_NODE_UNLISTED);
> + return node;
> +}
> +
> +extern bool llist_add_exclusive(struct llist_node *, struct llist_head *);
> +
> #endif /* LLIST_H */
> diff --git a/lib/llist.c b/lib/llist.c
> index 7062e93..c85fa6b 100644
> --- a/lib/llist.c
> +++ b/lib/llist.c
> @@ -102,3 +102,32 @@ struct llist_node *llist_reverse_order(struct llist_node *head)
> return new_head;
> }
> EXPORT_SYMBOL_GPL(llist_reverse_order);
> +
> +/**
> + * llist_add_exclusive - add a node only if its ->next is NULL
> + * @node: the node to be added
> + * @head: the head for your lock-less list
> + *
> + * Return true if the node was added, or false otherwise.
> + */
> +bool llist_add_exclusive(struct llist_node *node, struct llist_head *head)
> +{
> + struct llist_node *next = LLIST_NODE_UNLISTED;
> + struct llist_node *entry, *new_next;
> +
> + /*
> + * if new_next is next (== LLIST_NODE_UNLISTED) on the first run, we
> + * get exclusive access to this node as long as others use
> + * llist_add_safe too.
> + * Then false can be returned no more and we'll loop until we get the
> + * stuff right.
> + */
> + do {
> + entry = READ_ONCE(head->first);
> + if ((new_next = cmpxchg(&node->next, next, entry)) != next)
> + return false;
> + next = entry;
> + } while (cmpxchg(&head->first, entry, node) != entry);
> + return true;
> +}
> +EXPORT_SYMBOL_GPL(llist_add_exclusive);
I think this could be implemented on top of llist, why add it into llist
itself?
Best Regards,
Huang, Ying
Vitaly Wool <[email protected]> writes:
> 2017-12-19 2:35 GMT+01:00 Huang, Ying <[email protected]>:
>
>> Vitaly Wool <[email protected]> writes:
>>
>> > It sometimes is necessary to be able to be able to use llist in
>> > the following manner:
>> > if (node_unlisted(node))
>> > llst_add(node, list);
>> > i. e. only add a node to the list if it's not already on a list.
>> >
>> > This is not possible without taking locks because otherwise there's
>> > an obvious race between the check and the addition. This patch adds
>> > the routine to add a node only if it is not on any list that is
>> > lockless and race free as long as there's only one consumer.
>> >
>> > That is, llist_add_exclusive would only add a node if it's not
>> > already on a list, and llist_del_first_exclusive will delete the
>> > first node off the list and mark it as not being on any list.
>> >
>> > Signed-off-by: Vitaly Wool <[email protected]>
>> > ---
>> > include/linux/llist.h | 25 +++++++++++++++++++++++++
>> > lib/llist.c | 29 +++++++++++++++++++++++++++++
>> > 2 files changed, 54 insertions(+)
>> >
>> > diff --git a/include/linux/llist.h b/include/linux/llist.h
>> > index 85abc29..5c60e9e 100644
>> > --- a/include/linux/llist.h
>> > +++ b/include/linux/llist.h
>> > @@ -74,6 +74,10 @@ struct llist_node {
>> > #define LLIST_HEAD_INIT(name) { NULL }
>> > #define LLIST_HEAD(name) struct llist_head name =
>> LLIST_HEAD_INIT(name)
>> >
>> > +#define LLIST_NODE_UNLISTED ((void *)(-1L))
>> > +#define LLIST_NODE_INIT(name) { LLIST_NODE_UNLISTED }
>> > +#define LLIST_NODE(name) struct llist_node name =
>> LLIST_NODE_INIT(name)
>> > +
>> > /**
>> > * init_llist_head - initialize lock-less list head
>> > * @head: the head for your lock-less list
>> > @@ -238,4 +242,25 @@ extern struct llist_node *llist_del_first(struct
>> llist_head *head);
>> >
>> > struct llist_node *llist_reverse_order(struct llist_node *head);
>> >
>> > +/**
>> > + * llist_del_first_exclusive - delete the first entry of lock-less list
>> > + * and make sure its ->next is NULL
>> > + * @head: the head for your lock-less list
>> > + *
>> > + * If list is empty, return NULL, otherwise, return the first entry
>> > + * deleted, this is the newest added one.
>> > + *
>> > + */
>> > +static inline struct llist_node *llist_del_first_exclusive(
>> > + struct llist_head *head)
>> > +{
>> > + struct llist_node *node = llist_del_first(head);
>> > +
>> > + if (READ_ONCE(node))
>> > + smp_store_release(&node->next, LLIST_NODE_UNLISTED);
>> > + return node;
>> > +}
>> > +
>> > +extern bool llist_add_exclusive(struct llist_node *, struct llist_head
>> *);
>> > +
>> > #endif /* LLIST_H */
>> > diff --git a/lib/llist.c b/lib/llist.c
>> > index 7062e93..c85fa6b 100644
>> > --- a/lib/llist.c
>> > +++ b/lib/llist.c
>> > @@ -102,3 +102,32 @@ struct llist_node *llist_reverse_order(struct
>> llist_node *head)
>> > return new_head;
>> > }
>> > EXPORT_SYMBOL_GPL(llist_reverse_order);
>> > +
>> > +/**
>> > + * llist_add_exclusive - add a node only if its ->next is NULL
>> > + * @node: the node to be added
>> > + * @head: the head for your lock-less list
>> > + *
>> > + * Return true if the node was added, or false otherwise.
>> > + */
>> > +bool llist_add_exclusive(struct llist_node *node, struct llist_head
>> *head)
>> > +{
>> > + struct llist_node *next = LLIST_NODE_UNLISTED;
>> > + struct llist_node *entry, *new_next;
>> > +
>> > + /*
>> > + * if new_next is next (== LLIST_NODE_UNLISTED) on the first run,
>> we
>> > + * get exclusive access to this node as long as others use
>> > + * llist_add_safe too.
>> > + * Then false can be returned no more and we'll loop until we get
>> the
>> > + * stuff right.
>> > + */
>> > + do {
>> > + entry = READ_ONCE(head->first);
>> > + if ((new_next = cmpxchg(&node->next, next, entry)) != next)
>> > + return false;
>> > + next = entry;
>> > + } while (cmpxchg(&head->first, entry, node) != entry);
>> > + return true;
>> > +}
>> > +EXPORT_SYMBOL_GPL(llist_add_exclusive);
>>
>> I think this could be implemented on top of llist, why add it into llist
>> itself?
>>
>
>
> Could you please elaborate how this would be implemented "on top"?
struct llist_node *my_del_first_exclusive(struct llist_head *head)
{
struct llist_node *node = llist_del_first(head);
if (node)
node->next = LLIST_NODE_UNLISTED;
}
bool my_add_exclusive(struct llist_node *node, struct llist_head *head)
{
if (node->next != LLIST_NODE_UNLIST)
return false;
if (cmpxchg(&node->next, LLIST_NODE_UNLIST, NULL) !=
LLIST_NODE_UNLIST)
return false;
llist_add(node, head);
return true;
}
Best Regards,
Huang, Ying
Vitaly Wool <[email protected]> writes:
> 2017-12-20 1:57 GMT+01:00 Huang, Ying <[email protected]>:
>
> <snip>
>
>>
>> > Could you please elaborate how this would be implemented "on top"?
>>
>> struct llist_node *my_del_first_exclusive(struct llist_head *head)
>> {
>> struct llist_node *node = llist_del_first(head);
>>
>> if (node)
>> node->next = LLIST_NODE_UNLISTED;
>> }
>>
>> bool my_add_exclusive(struct llist_node *node, struct llist_head *head)
>> {
>> if (node->next != LLIST_NODE_UNLIST)
>> return false;
>> if (cmpxchg(&node->next, LLIST_NODE_UNLIST, NULL) !=
>> LLIST_NODE_UNLIST)
>> return false;
>> llist_add(node, head);
>> return true;
>> }
>>
>
> That would work, thanks. I'll update the patch.
Is there any other users except your code? If no, I think it's better
to keep it with its only user instead part of llist library at least for
now. Because I don't know whether the usage model is popular.
Best Regards,
Huang, Ying
> Thanks,
> Vitaly
Vitaly Wool <[email protected]> writes:
> 2017-12-22 14:57 GMT+01:00 Huang, Ying <[email protected]>:
>
>> Vitaly Wool <[email protected]> writes:
>>
>> > 2017-12-20 1:57 GMT+01:00 Huang, Ying <[email protected]>:
>> >
>> > <snip>
>> >
>> >>
>> >> > Could you please elaborate how this would be implemented "on top"?
>> >>
>> >> struct llist_node *my_del_first_exclusive(struct llist_head *head)
>> >> {
>> >> struct llist_node *node = llist_del_first(head);
>> >>
>> >> if (node)
>> >> node->next = LLIST_NODE_UNLISTED;
>> >> }
>> >>
>> >> bool my_add_exclusive(struct llist_node *node, struct llist_head *head)
>> >> {
>> >> if (node->next != LLIST_NODE_UNLIST)
>> >> return false;
>> >> if (cmpxchg(&node->next, LLIST_NODE_UNLIST, NULL) !=
>> >> LLIST_NODE_UNLIST)
>> >> return false;
>> >> llist_add(node, head);
>> >> return true;
>> >> }
>> >>
>> >
>> > That would work, thanks. I'll update the patch.
>>
>> Is there any other users except your code? If no, I think it's better
>> to keep it with its only user instead part of llist library at least for
>> now. Because I don't know whether the usage model is popular.
>>
>>
>>
> I'm going to come up with a patch to binder that uses these specific
> extensions, so I expect there'll be some users for this.
I think it is better to keep this in binder unless there are other
users.
Best Regards,
Huang, Ying
> Thanks,
> Vitaly