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Subject[PATCH v2 tip/core/rcu 6/6] srcu: Document polling interfaces for Tree SRCU grace periods
From: "Paul E. McKenney" <>

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

Reported-by: Kent Overstreet <>
Signed-off-by: Paul E. McKenney <>
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 1e3df77..2dce79d 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

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