doc: Update list_for_each_entry_rcu() documentation

This commit updates the documentation with information about
usage of lockdep with list_for_each_entry_rcu().

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
[ paulmck: Wordsmithing. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/Documentation/RCU/lockdep.txt b/Documentation/RCU/lockdep.txt
index da51d30..89db949 100644 (file)
--- a/Documentation/RCU/lockdep.txt
+++ b/Documentation/RCU/lockdep.txt
@@ -96,7 +96,17 @@ 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
-being in an RCU read-side critical section.  In the future, separate
-versions of these primitives might be created.
+Like rcu_dereference(), when lockdep is enabled, RCU list and hlist
+traversal primitives check for being called from within an RCU read-side
+critical section.  However, a lockdep expression can be passed to them
+as a additional optional argument.  With this lockdep expression, these
+traversal primitives will complain only if the lockdep expression is
+false and they are called from outside any RCU read-side critical section.
+
+For example, the workqueue for_each_pwq() macro is intended to be used
+either within an RCU read-side critical section or with wq->mutex held.
+It is thus implemented as follows:
+
+       #define for_each_pwq(pwq, wq)
+               list_for_each_entry_rcu((pwq), &(wq)->pwqs, pwqs_node,
+                                       lock_is_held(&(wq->mutex).dep_map))

diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index 17f4831..58ba05c 100644 (file)
--- a/Documentation/RCU/whatisRCU.txt
+++ b/Documentation/RCU/whatisRCU.txt
@@ -290,7 +290,7 @@ rcu_dereference()
        at any time, including immediately after the rcu_dereference().
        And, again like rcu_assign_pointer(), rcu_dereference() is
        typically used indirectly, via the _rcu list-manipulation
-       primitives, such as list_for_each_entry_rcu().
+       primitives, such as list_for_each_entry_rcu() [2].
 
        [1] The variant rcu_dereference_protected() can be used outside
        of an RCU read-side critical section as long as the usage is
@@ -305,6 +305,14 @@ rcu_dereference()
        a lockdep splat is emitted.  See Documentation/RCU/Design/Requirements/Requirements.rst
        and the API's code comments for more details and example usage.
 
+       [2] If the list_for_each_entry_rcu() instance might be used by
+       update-side code as well as by RCU readers, then an additional
+       lockdep expression can be added to its list of arguments.
+       For example, given an additional "lock_is_held(&mylock)" argument,
+       the RCU lockdep code would complain only if this instance was
+       invoked outside of an RCU read-side critical section and without
+       the protection of mylock.
+
 The following diagram shows how each API communicates among the
 reader, updater, and reclaimer.