[PATCH tip/core/rcu 06/10] srcu: Document polling interfaces for Tree SRCU grace periods

From: paulmck
Date: Wed Jan 06 2021 - 11:56:43 EST

From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>

This commit adds requirements documentation for the
get_state_synchronize_srcu(), start_poll_synchronize_srcu(), and
poll_state_synchronize_srcu() functions.

Link: https://lore.kernel.org/rcu/20201112201547.GF3365678@xxxxxxxxxxxxxx/
Reported-by: Kent Overstreet <kent.overstreet@xxxxxxxxx>
Reviewed-by: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Documentation/RCU/Design/Requirements/Requirements.rst | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)

diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst
index e8c84fc..93a189a 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.rst
+++ b/Documentation/RCU/Design/Requirements/Requirements.rst
@@ -2600,6 +2600,24 @@ also includes ``DEFINE_SRCU()``, ``DEFINE_STATIC_SRCU()``, and
``init_srcu_struct()`` APIs for defining and initializing
``srcu_struct`` structures.

+More recently, the SRCU API has added polling interfaces:
+#. start_poll_synchronize_srcu() returns a cookie identifying
+ the completion of a future SRCU grace period and ensures
+ that this grace period will be started.
+#. poll_state_synchronize_srcu() returns ``true`` iff the
+ specified cookie corresponds to an already-completed
+ SRCU grace period.
+#. get_state_synchronize_srcu() returns a cookie just like
+ start_poll_synchronize_srcu() does, but differs in that
+ it does nothing to ensure that any future SRCU grace period
+ will be started.
+These functions are used to avoid unnecessary SRCU grace periods in
+certain types of buffer-cache algorithms having multi-stage age-out
+mechanisms. The idea is that by the time the block has aged completely
+from the cache, an SRCU grace period will be very likely to have elapsed.
Tasks RCU