rcu: Documentation update

This commit applies a few updates based on a quick review of the RCU
documentations.

Signed-off-by: Paul E. McKenney <[e-mail]>

Documentation/RCU/checklist.txt

···
 	whether the increased speed is worth it.
 
 8.	Although synchronize_rcu() is slower than is call_rcu(), it
-	usually results in simpler code.  So, unless update performance
-	is critically important or the updaters cannot block,
-	synchronize_rcu() should be used in preference to call_rcu().
+	usually results in simpler code.  So, unless update performance is
+	critically important, the updaters cannot block, or the latency of
+	synchronize_rcu() is visible from userspace, synchronize_rcu()
+	should be used in preference to call_rcu().  Furthermore,
+	kfree_rcu() usually results in even simpler code than does
+	synchronize_rcu() without synchronize_rcu()'s multi-millisecond
+	latency.  So please take advantage of kfree_rcu()'s "fire and
+	forget" memory-freeing capabilities where it applies.
 
 	An especially important property of the synchronize_rcu()
 	primitive is that it automatically self-limits: if grace periods
···
 	e.	Periodically invoke synchronize_rcu(), permitting a limited
 		number of updates per grace period.
 
-	The same cautions apply to call_rcu_bh() and call_rcu_sched().
+	The same cautions apply to call_rcu_bh(), call_rcu_sched(),
+	call_srcu(), and kfree_rcu().
 
 9.	All RCU list-traversal primitives, which include
 	rcu_dereference(), list_for_each_entry_rcu(), and
···
 	all currently executing rcu_read_lock()-protected RCU read-side
 	critical sections complete.  It does -not- necessarily guarantee
 	that all currently running interrupts, NMIs, preempt_disable()
-	code, or idle loops will complete.  Therefore, if you do not have
-	rcu_read_lock()-protected read-side critical sections, do -not-
-	use synchronize_rcu().
+	code, or idle loops will complete.  Therefore, if your
+	read-side critical sections are protected by something other
+	than rcu_read_lock(), do -not- use synchronize_rcu().
 
 	Similarly, disabling preemption is not an acceptable substitute
 	for rcu_read_lock().  Code that attempts to use preemption
···
 	read-side critical sections.  It is the responsibility of the
 	RCU update-side primitives to deal with this.
 
-17.	Use CONFIG_PROVE_RCU, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and
-	the __rcu sparse checks to validate your RCU code.  These
-	can help find problems as follows:
+17.	Use CONFIG_PROVE_RCU, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and the
+	__rcu sparse checks (enabled by CONFIG_SPARSE_RCU_POINTER) to
+	validate your RCU code.  These can help find problems as follows:
 
 	CONFIG_PROVE_RCU: check that accesses to RCU-protected data
 		structures are carried out under the proper RCU

Documentation/RCU/lockdep.txt

···
 		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.
+	rcu_access_index(idx):
+		Return the value of the index and omit all barriers, but
+		retain the compiler constraints that prevent duplicating
+		or coalescsing.  This is useful when when testing the
+		value of the index itself, for example, against -1.
 
 The rcu_dereference_check() check expression can be any boolean
 expression, but would normally include a lockdep expression.  However,

Documentation/RCU/rcubarrier.txt

···
    2. Execute rcu_barrier().
    3. Allow the module to be unloaded.
 
-The rcutorture module makes use of rcu_barrier in its exit function
+There are also rcu_barrier_bh(), rcu_barrier_sched(), and srcu_barrier()
+functions for the other flavors of RCU, and you of course must match
+the flavor of rcu_barrier() with that of call_rcu().  If your module
+uses multiple flavors of call_rcu(), then it must also use multiple
+flavors of rcu_barrier() when unloading that module.  For example, if
+it uses call_rcu_bh(), call_srcu() on srcu_struct_1, and call_srcu() on
+srcu_struct_2(), then the following three lines of code will be required
+when unloading:
+
+ 1 rcu_barrier_bh();
+ 2 srcu_barrier(&srcu_struct_1);
+ 3 srcu_barrier(&srcu_struct_2);
+
+The rcutorture module makes use of rcu_barrier() in its exit function
 as follows:
 
  1 static void