From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx> More explicitly state what is, and what is not guaranteed to those who iterate a list while protected by RCU. [ paulmck: Apply Joel Fernandes feedback. ] Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx> Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx> --- Documentation/RCU/listRCU.rst | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/Documentation/RCU/listRCU.rst b/Documentation/RCU/listRCU.rst index bdc4bcc5289f..ed5c9d8c9afe 100644 --- a/Documentation/RCU/listRCU.rst +++ b/Documentation/RCU/listRCU.rst @@ -8,6 +8,15 @@ One of the most common uses of RCU is protecting read-mostly linked lists that all of the required memory ordering is provided by the list macros. This document describes several list-based RCU use cases. +When iterating a list while holding the rcu_read_lock(), writers may +modify the list. The reader is guaranteed to see all of the elements +which were added to the list before they acquired the rcu_read_lock() +and are still on the list when they drop the rcu_read_unlock(). +Elements which are added to, or removed from the list may or may not +be seen. If the writer calls list_replace_rcu(), the reader may see +either the old element or the new element; they will not see both, +nor will they see neither. + Example 1: Read-mostly list: Deferred Destruction ------------------------------------------------- -- 2.34.1
