From d84297c99bd9f63baf92c9f2d36582f879858664 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Thu, 18 Apr 2013 14:25:03 -0700 Subject: 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 Reported-by: Johannes Berg Signed-off-by: Paul E. McKenney Reviewed-by: Josh Triplett --- Documentation/RCU/rcubarrier.txt | 12 ++++++++---- 1 file changed, 8 insertions(+), 4 deletions(-) (limited to 'Documentation/RCU/rcubarrier.txt') diff --git a/Documentation/RCU/rcubarrier.txt b/Documentation/RCU/rcubarrier.txt index 2e319d1b9ef2..b10cfe711e68 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(). -- cgit v1.2.3