1 files changed, 17 insertions, 2 deletions

diff --git a/Documentation/RCU/rcu.txt b/Documentation/RCU/rcu.txt
index f84407cba816..95821a29ae41 100644
--- a/Documentation/RCU/rcu.txt
+++ b/Documentation/RCU/rcu.txt
@@ -36,6 +36,14 @@ o	How can the updater tell when a grace period has completed
         executed in user mode, or executed in the idle loop, we can
         safely free up that item.
 
+        Preemptible variants of RCU (CONFIG_PREEMPT_RCU) get the
+        same effect, but require that the readers manipulate CPU-local
+        counters.  These counters allow limited types of blocking
+        within RCU read-side critical sections.  SRCU also uses
+        CPU-local counters, and permits general blocking within
+        RCU read-side critical sections.  These two variants of
+        RCU detect grace periods by sampling these counters.
+
 o       If I am running on a uniprocessor kernel, which can only do one
         thing at a time, why should I wait for a grace period?
 
@@ -46,7 +54,10 @@ o	How can I see where RCU is currently used in the Linux kernel?
         Search for "rcu_read_lock", "rcu_read_unlock", "call_rcu",
         "rcu_read_lock_bh", "rcu_read_unlock_bh", "call_rcu_bh",
         "srcu_read_lock", "srcu_read_unlock", "synchronize_rcu",
-        "synchronize_net", and "synchronize_srcu".
+        "synchronize_net", "synchronize_srcu", and the other RCU
+        primitives.  Or grab one of the cscope databases from:
+
+        http://www.rdrop.com/users/paulmck/RCU/linuxusage/rculocktab.html
 
 o       What guidelines should I follow when writing code that uses RCU?
 
@@ -67,7 +78,11 @@ o	I hear that RCU is patented?  What is with that?
 
 o       I hear that RCU needs work in order to support realtime kernels?
 
-        Yes, work in progress.
+        This work is largely completed.  Realtime-friendly RCU can be
+        enabled via the CONFIG_PREEMPT_RCU kernel configuration parameter.
+        However, work is in progress for enabling priority boosting of
+        preempted RCU read-side critical sections.This is needed if you
+        have CPU-bound realtime threads.
 
 o       Where can I find more information on RCU?