[tip: core/rcu] doc: Update RCU requirements RCU_INIT_POINTER() description

[tip-bot2 for Paul E. McKenney]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     d756c74e6f6e76e99f8bffcea57833816dd335b6
Gitweb:        https://git.kernel.org/tip/d756c74e6f6e76e99f8bffcea57833816dd335b6
Author:        Paul E. McKenney <paulmck@xxxxxxxxxx>
AuthorDate:    Wed, 09 Dec 2020 16:54:41 -08:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Mon, 04 Jan 2021 13:35:15 -08:00

doc: Update RCU requirements RCU_INIT_POINTER() description

Back in the day, RCU_INIT_POINTER() was the only way to avoid
memory-barrier instructions while storing NULL to an RCU-protected
pointer.  Fortunately, in 2016, rcu_assign_pointer() started checking for
compile-time NULL pointers and omitting the memory-barrier instructions in
that case.  Unfortunately, RCU's Requirements.rst document was not updated
accordingly.  This commit therefore at long last carries out that update.

Fixes: 3a37f7275cda ("rcu: No ordering for rcu_assign_pointer() of NULL")
Link: https://lore.kernel.org/lkml/20201209230755.GV7338@xxxxxxxxxxxxxxxxxxxx/
Reported-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Acked-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 Documentation/RCU/Design/Requirements/Requirements.rst | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst
index f32f8fa..65c7839 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.rst
+++ b/Documentation/RCU/Design/Requirements/Requirements.rst
@@ -1668,8 +1668,7 @@ against mishaps and misuse:
    this purpose.
 #. It is not necessary to use rcu_assign_pointer() when creating
    linked structures that are to be published via a single external
-   pointer. The RCU_INIT_POINTER() macro is provided for this task
-   and also for assigning ``NULL`` pointers at runtime.
+   pointer. The RCU_INIT_POINTER() macro is provided for this task.
 
 This not a hard-and-fast list: RCU's diagnostic capabilities will
 continue to be guided by the number and type of usage bugs found in