[PATCH tip/core/rcu 7/7] rcu: add comment stating that list_empty() applies to RCU-protected lists

[Paul E. McKenney]

Because list_empty() does not dereference any RCU-protected pointers, and
further does not pass such pointers to the caller (so that the caller
does not dereference them either), it is safe to use list_empty() on
RCU-protected lists.  There is no need for a list_empty_rcu().  This
commit adds a comment stating this explicitly.

Requested-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
 include/linux/rculist.h |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)

diff --git a/include/linux/rculist.h b/include/linux/rculist.h
index c10b105..f31ef61 100644
--- a/include/linux/rculist.h
+++ b/include/linux/rculist.h
@@ -10,6 +10,15 @@
 #include <linux/rcupdate.h>
 
 /*
+ * Why is there no list_empty_rcu()?  Because list_empty() serves this
+ * purpose.  The list_empty() function fetches the RCU-protected pointer
+ * and compares it to the address of the list head, but neither dereferences
+ * this pointer itself nor provides this pointer to the caller.  Therefore,
+ * it is not necessary to use rcu_dereference(), so that list_empty() can
+ * be used anywhere you would want to use a list_empty_rcu().
+ */
+
+/*
  * return the ->next pointer of a list_head in an rcu safe
  * way, we must not access it directly
  */
-- 
1.7.0.6

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