[PATCH] Yet another RCU documentation update
Update RCU documentation based on discussions and review of RCU-based tree
patches. Add an introductory whatisRCU.txt file.
Signed-off-by: <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt
index 8f3fb77..e118a7c 100644
--- a/Documentation/RCU/checklist.txt
+++ b/Documentation/RCU/checklist.txt
@@ -43,6 +43,10 @@
rcu_read_lock_bh()) in the read-side critical sections,
and are also an excellent aid to readability.
+ As a rough rule of thumb, any dereference of an RCU-protected
+ pointer must be covered by rcu_read_lock() or rcu_read_lock_bh()
+ or by the appropriate update-side lock.
+
3. Does the update code tolerate concurrent accesses?
The whole point of RCU is to permit readers to run without
@@ -90,7 +94,11 @@
The rcu_dereference() primitive is used by the various
"_rcu()" list-traversal primitives, such as the
- list_for_each_entry_rcu().
+ list_for_each_entry_rcu(). Note that it is perfectly
+ legal (if redundant) for update-side code to use
+ rcu_dereference() and the "_rcu()" list-traversal
+ primitives. This is particularly useful in code
+ that is common to readers and updaters.
b. If the list macros are being used, the list_add_tail_rcu()
and list_add_rcu() primitives must be used in order
@@ -150,16 +158,9 @@
Use of the _rcu() list-traversal primitives outside of an
RCU read-side critical section causes no harm other than
- a slight performance degradation on Alpha CPUs and some
- confusion on the part of people trying to read the code.
-
- Another way of thinking of this is "If you are holding the
- lock that prevents the data structure from changing, why do
- you also need RCU-based protection?" That said, there may
- well be situations where use of the _rcu() list-traversal
- primitives while the update-side lock is held results in
- simpler and more maintainable code. The jury is still out
- on this question.
+ a slight performance degradation on Alpha CPUs. It can
+ also be quite helpful in reducing code bloat when common
+ code is shared between readers and updaters.
10. Conversely, if you are in an RCU read-side critical section,
you -must- use the "_rcu()" variants of the list macros.