Commit-ID: d84297c99bd9f63baf92c9f2d36582f879858664
Gitweb: http://git.kernel.org/tip/d84297c99bd9f63baf92c9f2d36582f879858664
Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
AuthorDate: Thu, 18 Apr 2013 14:25:03 -0700
Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
CommitDate: Sun, 18 Aug 2013 17:05:32 -0700
rcu: Fix rcu_barrier() documentation
There was a time when rcu_barrier() was guaranteed to wait for at least
a grace period, but that time ended due to energy-efficiency concerns.
So now rcu_barrier() is a no-op if there are no RCU callbacks queued in
the system. This commit updates the documentation to reflect this change.
Now, rcu_barrier() often does wait for a grace period, so, one could
imagine some modification to rcu_barrier() to more efficiently handle
cases where both rcu_barrier() and a grace period are needed. But this
must wait until someone shows a real-world need for a change.
Reported-by: Bob Copeland <bob@xxxxxxxxxxx>
Reported-by: Johannes Berg <johannes@xxxxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
---
Documentation/RCU/rcubarrier.txt | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)
diff --git a/Documentation/RCU/rcubarrier.txt b/Documentation/RCU/rcubarrier.txt
index 2e319d1..b10cfe7 100644
--- a/Documentation/RCU/rcubarrier.txt
+++ b/Documentation/RCU/rcubarrier.txt
@@ -70,10 +70,14 @@ in realtime kernels in order to avoid excessive scheduling latencies.
rcu_barrier()
-We instead need the rcu_barrier() primitive. This primitive is similar
-to synchronize_rcu(), but instead of waiting solely for a grace
-period to elapse, it also waits for all outstanding RCU callbacks to
-complete. Pseudo-code using rcu_barrier() is as follows:
+We instead need the rcu_barrier() primitive. Rather than waiting for
+a grace period to elapse, rcu_barrier() waits for all outstanding RCU
+callbacks to complete. Please note that rcu_barrier() does -not- imply
+synchronize_rcu(), in particular, if there are no RCU callbacks queued
+anywhere, rcu_barrier() is within its rights to return immediately,
+without waiting for a grace period to elapse.
+
+Pseudo-code using rcu_barrier() is as follows:
1. Prevent any new RCU callbacks from being posted.
2. Execute rcu_barrier().
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