Document the fact that RCU callbacks can run in parallel

Add an item to the RCU documentation checklist noting that RCU callbacks
can run in parallel.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 1 deletions

diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt
index f4dffadbcb00..42b01bc2e1b4 100644
--- a/Documentation/RCU/checklist.txt
+++ b/Documentation/RCU/checklist.txt
@@ -222,7 +222,15 @@ over a rather long period of time, but improvements are always welcome!
         deadlock as soon as the RCU callback happens to interrupt that
         acquisition's critical section.
 
-13.     SRCU (srcu_read_lock(), srcu_read_unlock(), and synchronize_srcu())
+13.     RCU callbacks can be and are executed in parallel.  In many cases,
+        the callback code simply wrappers around kfree(), so that this
+        is not an issue (or, more accurately, to the extent that it is
+        an issue, the memory-allocator locking handles it).  However,
+        if the callbacks do manipulate a shared data structure, they
+        must use whatever locking or other synchronization is required
+        to safely access and/or modify that data structure.
+
+14.     SRCU (srcu_read_lock(), srcu_read_unlock(), and synchronize_srcu())
         may only be invoked from process context.  Unlike other forms of
         RCU, it -is- permissible to block in an SRCU read-side critical
         section (demarked by srcu_read_lock() and srcu_read_unlock()),