rcu: Documentation update

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

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

3 files changed, 35 insertions, 11 deletions

diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt
index 31ef8fe07f82..79e789b8b8ea 100644
--- a/Documentation/RCU/checklist.txt
+++ b/Documentation/RCU/checklist.txt
@@ -217,9 +217,14 @@ over a rather long period of time, but improvements are always welcome!
         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
@@ -268,7 +273,8 @@ over a rather long period of time, but improvements are always welcome!
         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
@@ -296,9 +302,9 @@ over a rather long period of time, but improvements are always welcome!
         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
@@ -401,9 +407,9 @@ over a rather long period of time, but improvements are always welcome!
         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
diff --git a/Documentation/RCU/lockdep.txt b/Documentation/RCU/lockdep.txt
index a102d4b3724b..cd83d2348fef 100644
--- a/Documentation/RCU/lockdep.txt
+++ b/Documentation/RCU/lockdep.txt
@@ -64,6 +64,11 @@ checking of rcu_dereference() primitives:
                 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,
diff --git a/Documentation/RCU/rcubarrier.txt b/Documentation/RCU/rcubarrier.txt
index 38428c125135..2e319d1b9ef2 100644
--- a/Documentation/RCU/rcubarrier.txt
+++ b/Documentation/RCU/rcubarrier.txt
@@ -79,7 +79,20 @@ complete. Pseudo-code using rcu_barrier() is as follows:
    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