rcu: Document unique-name limitation for DEFINE_STATIC_SRCU()

SRCU uses per-CPU variables, and DEFINE_STATIC_SRCU() uses a static
per-CPU variable.  However, per-CPU variables have significant
restrictions, for example, names of per-CPU variables must be globally
unique, even if declared static.  These restrictions carry over to
DEFINE_STATIC_SRCU(), and this commit therefore documents these
restrictions.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Suggested-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Tejun Heo <tj@kernel.org>

include/linux/srcu.h

@@ -99,8 +99,23 @@ void process_srcu(struct work_struct *work);
 	}
 
 /*
- * define and init a srcu struct at build time.
- * dont't call init_srcu_struct() nor cleanup_srcu_struct() on it.
+ * Define and initialize a srcu struct at build time.
+ * Do -not- call init_srcu_struct() nor cleanup_srcu_struct() on it.
+ *
+ * Note that although DEFINE_STATIC_SRCU() hides the name from other
+ * files, the per-CPU variable rules nevertheless require that the
+ * chosen name be globally unique.  These rules also prohibit use of
+ * DEFINE_STATIC_SRCU() within a function.  If these rules are too
+ * restrictive, declare the srcu_struct manually.  For example, in
+ * each file:
+ *
+ *	static struct srcu_struct my_srcu;
+ *
+ * Then, before the first use of each my_srcu, manually initialize it:
+ *
+ *	init_srcu_struct(&my_srcu);
+ *
+ * See include/linux/percpu-defs.h for the rules on per-CPU variables.
  */
 #define __DEFINE_SRCU(name, is_static)					\
 	static DEFINE_PER_CPU(struct srcu_struct_array, name##_srcu_array);\