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>

diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt
index 6e25340..accfe2f 100644 (file)
--- 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.