rcu: documentation 1Q09 update

Update the RCU documentation to call out the need for callers of
primitives like call_rcu() and synchronize_rcu() to prevent subsequent RCU
readers from hazard.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt
index 6e253407b3dc..accfe2f5247d 100644
--- a/Documentation/RCU/checklist.txt
+++ b/Documentation/RCU/checklist.txt
@@ -298,3 +298,15 @@ over a rather long period of time, but improvements are always welcome!
 
         Note that, rcu_assign_pointer() and rcu_dereference() relate to
         SRCU just as they do to other forms of RCU.
+
+15.     The whole point of call_rcu(), synchronize_rcu(), and friends
+        is to wait until all pre-existing readers have finished before
+        carrying out some otherwise-destructive operation.  It is
+        therefore critically important to -first- remove any path
+        that readers can follow that could be affected by the
+        destructive operation, and -only- -then- invoke call_rcu(),
+        synchronize_rcu(), or friends.
+
+        Because these primitives only wait for pre-existing readers,
+        it is the caller's responsibility to guarantee safety to
+        any subsequent readers.