[tip:sched/core] llist: Provide a safe version for llist_for_each()

[tip-bot for Byungchul Park]

Commit-ID:  d714893e61cd8c6e5c7e095f7dd615aa434bca95
Gitweb:     http://git.kernel.org/tip/d714893e61cd8c6e5c7e095f7dd615aa434bca95
Author:     Byungchul Park <byungchul.park@xxxxxxx>
AuthorDate: Fri, 12 May 2017 09:36:56 +0900
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 23 May 2017 10:01:33 +0200

llist: Provide a safe version for llist_for_each()

Sometimes we have to dereference next field of llist node before entering
loop becasue the node might be deleted or the next field might be
modified within the loop. So this adds the safe version of llist_for_each(),
that is, llist_for_each_safe().

Signed-off-by: Byungchul Park <byungchul.park@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Huang, Ying <ying.huang@xxxxxxxxx>
Cc: <kernel-team@xxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/1494549416-10539-1-git-send-email-byungchul.park@xxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 include/linux/llist.h | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/include/linux/llist.h b/include/linux/llist.h
index 171baa9..d117381 100644
--- a/include/linux/llist.h
+++ b/include/linux/llist.h
@@ -110,6 +110,25 @@ static inline void init_llist_head(struct llist_head *list)
 	for ((pos) = (node); pos; (pos) = (pos)->next)
 
 /**
+ * llist_for_each_safe - iterate over some deleted entries of a lock-less list
+ *			 safe against removal of list entry
+ * @pos:	the &struct llist_node to use as a loop cursor
+ * @n:		another &struct llist_node to use as temporary storage
+ * @node:	the first entry of deleted list entries
+ *
+ * In general, some entries of the lock-less list can be traversed
+ * safely only after being deleted from list, so start with an entry
+ * instead of list head.
+ *
+ * If being used on entries deleted from lock-less list directly, the
+ * traverse order is from the newest to the oldest added entry.  If
+ * you want to traverse from the oldest to the newest, you must
+ * reverse the order by yourself before traversing.
+ */
+#define llist_for_each_safe(pos, n, node)			\
+	for ((pos) = (node); (pos) && ((n) = (pos)->next, true); (pos) = (n))
+
+/**
  * llist_for_each_entry - iterate over some deleted entries of lock-less list of given type
  * @pos:	the type * to use as a loop cursor.
  * @node:	the fist entry of deleted list entries.