rcu: document ways of stalling updates in low-memory situations

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt
index c7c6788956f40a3428f677b69874dded3b7d8a2b..0c134f8afc6f60b1316b9551577179f1b6dc3961 100644 (file)
--- a/Documentation/RCU/checklist.txt
+++ b/Documentation/RCU/checklist.txt
@@ -218,13 +218,22 @@ over a rather long period of time, but improvements are always welcome!
        include:
 
        a.      Keeping a count of the number of data-structure elements
-               used by the RCU-protected data structure, including those
-               waiting for a grace period to elapse.  Enforce a limit
-               on this number, stalling updates as needed to allow
-               previously deferred frees to complete.
-
-               Alternatively, limit only the number awaiting deferred
-               free rather than the total number of elements.
+               used by the RCU-protected data structure, including
+               those waiting for a grace period to elapse.  Enforce a
+               limit on this number, stalling updates as needed to allow
+               previously deferred frees to complete.  Alternatively,
+               limit only the number awaiting deferred free rather than
+               the total number of elements.
+
+               One way to stall the updates is to acquire the update-side
+               mutex.  (Don't try this with a spinlock -- other CPUs
+               spinning on the lock could prevent the grace period
+               from ever ending.)  Another way to stall the updates
+               is for the updates to use a wrapper function around
+               the memory allocator, so that this wrapper function
+               simulates OOM when there is too much memory awaiting an
+               RCU grace period.  There are of course many other
+               variations on this theme.
 
        b.      Limiting update rate.  For example, if updates occur only
                once per hour, then no explicit rate limiting is required,