Diffstat (limited to 'Documentation/RCU/lockdep.txt')
1 files changed, 26 insertions, 2 deletions
diff --git a/Documentation/RCU/lockdep.txt b/Documentation/RCU/lockdep.txt
index fe24b58627bd..d7a49b2f6994 100644
@@ -32,9 +32,20 @@ checking of rcu_dereference() primitives:
Check for SRCU read-side critical section.
- Use explicit check expression "c".
+ Use explicit check expression "c". This is useful in
+ code that is invoked by both readers and updaters.
Don't check. (Use sparingly, if at all.)
+ rcu_dereference_protected(p, c):
+ Use explicit check expression "c", and omit all barriers
+ and compiler constraints. This is useful when the data
+ structure cannot change, for example, in code that is
+ invoked only by updaters.
+ Return the value of the pointer and omit all barriers,
+ but retain the compiler constraints that prevent duplicating
+ or coalescsing. This is useful when when testing the
+ value of the pointer itself, for example, against NULL.
The rcu_dereference_check() check expression can be any boolean
expression, but would normally include one of the rcu_read_lock_held()
@@ -59,7 +70,20 @@ In case (1), the pointer is picked up in an RCU-safe manner for vanilla
RCU read-side critical sections, in case (2) the ->file_lock prevents
any change from taking place, and finally, in case (3) the current task
is the only task accessing the file_struct, again preventing any change
-from taking place.
+from taking place. If the above statement was invoked only from updater
+code, it could instead be written as follows:
+ file = rcu_dereference_protected(fdt->fd[fd],
+ lockdep_is_held(&files->file_lock) ||
+ atomic_read(&files->count) == 1);
+This would verify cases #2 and #3 above, and furthermore lockdep would
+complain if this was used in an RCU read-side critical section unless one
+of these two cases held. Because rcu_dereference_protected() omits all
+barriers and compiler constraints, it generates better code than do the
+other flavors of rcu_dereference(). On the other hand, it is illegal
+to use rcu_dereference_protected() if either the RCU-protected pointer
+or the RCU-protected data that it points to can change concurrently.
There are currently only "universal" versions of the rcu_assign_pointer()
and RCU list-/tree-traversal primitives, which do not (yet) check for