rcu: define __rcu address space modifier for sparse

This commit provides definitions for the __rcu annotation defined earlier.
This annotation permits sparse to check for correct use of RCU-protected
pointers.  If a pointer that is annotated with __rcu is accessed
directly (as opposed to via rcu_dereference(), rcu_assign_pointer(),
or one of their variants), sparse can be made to complain.  To enable
such complaints, use the new default-disabled CONFIG_SPARSE_RCU_POINTER
kernel configuration option.  Please note that these sparse complaints are
intended to be a debugging aid, -not- a code-style-enforcement mechanism.

There are special rcu_dereference_protected() and rcu_access_pointer()
accessors for use when RCU read-side protection is not required, for
example, when no other CPU has access to the data structure in question
or while the current CPU hold the update-side lock.

This patch also updates a number of docbook comments that were showing
their age.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Christopher Li <sparse@chrisli.org>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 1b4afd2e6ca0..12465f2ef766 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -539,6 +539,19 @@ config PROVE_RCU_REPEATEDLY
          disabling, allowing multiple RCU-lockdep warnings to be printed
          on a single reboot.
 
+config SPARSE_RCU_POINTER
+        bool "RCU debugging: sparse-based checks for pointer usage"
+        default n
+        help
+         This feature enables the __rcu sparse annotation for
+         RCU-protected pointers.  This annotation will cause sparse
+         to flag any non-RCU used of annotated pointers.  This can be
+         helpful when debugging RCU usage.  Please note that this feature
+         is not intended to enforce code cleanliness; it is instead merely
+         a debugging aid.
+
+         Say Y to make sparse flag questionable use of RCU-protected pointers
+
          Say N if you are unsure.
 
 config LOCKDEP