rcu: Update rcu_access_pointer() header for rcu_dereference_protected()

The rcu_access_pointer() docbook header correctly notes that it may be
used during post-grace-period teardown.  However, it is usually better to
use rcu_dereference_protected() for this purpose.  This commit therefore
calls out this preferred usage.

Reported-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

include/linux/rcupdate.h

@@ -496,13 +496,21 @@ do {									      \
  * against NULL.  Although rcu_access_pointer() may also be used in cases
  * where update-side locks prevent the value of the pointer from changing,
  * you should instead use rcu_dereference_protected() for this use case.
+ * Within an RCU read-side critical section, there is little reason to
+ * use rcu_access_pointer().
+ *
+ * It is usually best to test the rcu_access_pointer() return value
+ * directly in order to avoid accidental dereferences being introduced
+ * by later inattentive changes.  In other words, assigning the
+ * rcu_access_pointer() return value to a local variable results in an
+ * accident waiting to happen.
  *
  * It is also permissible to use rcu_access_pointer() when read-side
- * access to the pointer was removed at least one grace period ago, as
- * is the case in the context of the RCU callback that is freeing up
- * the data, or after a synchronize_rcu() returns.  This can be useful
- * when tearing down multi-linked structures after a grace period
- * has elapsed.
+ * access to the pointer was removed at least one grace period ago, as is
+ * the case in the context of the RCU callback that is freeing up the data,
+ * or after a synchronize_rcu() returns.  This can be useful when tearing
+ * down multi-linked structures after a grace period has elapsed.  However,
+ * rcu_dereference_protected() is normally preferred for this use case.
  */
 #define rcu_access_pointer(p) __rcu_access_pointer((p), __UNIQUE_ID(rcu), __rcu)