5 files changed, 95 insertions, 7 deletions

diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index cfaac34c4557..6ef692667e2f 100644
--- a/Documentation/RCU/whatisRCU.txt
+++ b/Documentation/RCU/whatisRCU.txt
@@ -849,6 +849,37 @@ All:  lockdep-checked RCU-protected pointer access
 See the comment headers in the source code (or the docbook generated
 from them) for more information.
 
+However, given that there are no fewer than four families of RCU APIs
+in the Linux kernel, how do you choose which one to use?  The following
+list can be helpful:
+
+a.      Will readers need to block?  If so, you need SRCU.
+
+b.      What about the -rt patchset?  If readers would need to block
+        in an non-rt kernel, you need SRCU.  If readers would block
+        in a -rt kernel, but not in a non-rt kernel, SRCU is not
+        necessary.
+
+c.      Do you need to treat NMI handlers, hardirq handlers,
+        and code segments with preemption disabled (whether
+        via preempt_disable(), local_irq_save(), local_bh_disable(),
+        or some other mechanism) as if they were explicit RCU readers?
+        If so, you need RCU-sched.
+
+d.      Do you need RCU grace periods to complete even in the face
+        of softirq monopolization of one or more of the CPUs?  For
+        example, is your code subject to network-based denial-of-service
+        attacks?  If so, you need RCU-bh.
+
+e.      Is your workload too update-intensive for normal use of
+        RCU, but inappropriate for other synchronization mechanisms?
+        If so, consider SLAB_DESTROY_BY_RCU.  But please be careful!
+
+f.      Otherwise, use RCU.
+
+Of course, this all assumes that you have determined that RCU is in fact
+the right tool for your job.
+
 
 8.  ANSWERS TO QUICK QUIZZES
 
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 631ad2f1b229..f0d3a8026a56 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -21,6 +21,7 @@ Contents:
      - SMP barrier pairing.
      - Examples of memory barrier sequences.
      - Read memory barriers vs load speculation.
+     - Transitivity
 
  (*) Explicit kernel barriers.
 
@@ -959,6 +960,63 @@ the speculation will be cancelled and the value reloaded:
         retrieved                               :       :       +-------+
 
 
+TRANSITIVITY
+------------
+
+Transitivity is a deeply intuitive notion about ordering that is not
+always provided by real computer systems.  The following example
+demonstrates transitivity (also called "cumulativity"):
+
+        CPU 1                   CPU 2                   CPU 3
+        ======================= ======================= =======================
+                { X = 0, Y = 0 }
+        STORE X=1               LOAD X                  STORE Y=1
+                                <general barrier>       <general barrier>
+                                LOAD Y                  LOAD X
+
+Suppose that CPU 2's load from X returns 1 and its load from Y returns 0.
+This indicates that CPU 2's load from X in some sense follows CPU 1's
+store to X and that CPU 2's load from Y in some sense preceded CPU 3's
+store to Y.  The question is then "Can CPU 3's load from X return 0?"
+
+Because CPU 2's load from X in some sense came after CPU 1's store, it
+is natural to expect that CPU 3's load from X must therefore return 1.
+This expectation is an example of transitivity: if a load executing on
+CPU A follows a load from the same variable executing on CPU B, then
+CPU A's load must either return the same value that CPU B's load did,
+or must return some later value.
+
+In the Linux kernel, use of general memory barriers guarantees
+transitivity.  Therefore, in the above example, if CPU 2's load from X
+returns 1 and its load from Y returns 0, then CPU 3's load from X must
+also return 1.
+
+However, transitivity is -not- guaranteed for read or write barriers.
+For example, suppose that CPU 2's general barrier in the above example
+is changed to a read barrier as shown below:
+
+        CPU 1                   CPU 2                   CPU 3
+        ======================= ======================= =======================
+                { X = 0, Y = 0 }
+        STORE X=1               LOAD X                  STORE Y=1
+                                <read barrier>          <general barrier>
+                                LOAD Y                  LOAD X
+
+This substitution destroys transitivity: in this example, it is perfectly
+legal for CPU 2's load from X to return 1, its load from Y to return 0,
+and CPU 3's load from X to return 0.
+
+The key point is that although CPU 2's read barrier orders its pair
+of loads, it does not guarantee to order CPU 1's store.  Therefore, if
+this example runs on a system where CPUs 1 and 2 share a store buffer
+or a level of cache, CPU 2 might have early access to CPU 1's writes.
+General barriers are therefore required to ensure that all CPUs agree
+on the combined order of CPU 1's and CPU 2's accesses.
+
+To reiterate, if your code requires transitivity, use general barriers
+throughout.
+
+
 ========================
 EXPLICIT KERNEL BARRIERS
 ========================
diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
index a23a57a976d1..f3240e987928 100644
--- a/kernel/rcupdate.c
+++ b/kernel/rcupdate.c
@@ -214,11 +214,12 @@ static int rcuhead_fixup_free(void *addr, enum debug_obj_state state)
                  * Ensure that queued callbacks are all executed.
                  * If we detect that we are nested in a RCU read-side critical
                  * section, we should simply fail, otherwise we would deadlock.
+                 * Note that the machinery to reliably determine whether
+                 * or not we are in an RCU read-side critical section
+                 * exists only in the preemptible RCU implementations
+                 * (TINY_PREEMPT_RCU and TREE_PREEMPT_RCU), which is why
+                 * DEBUG_OBJECTS_RCU_HEAD is disallowed if !PREEMPT.
                  */
-#ifndef CONFIG_PREEMPT
-                WARN_ON(1);
-                return 0;
-#else
                 if (rcu_preempt_depth() != 0 || preempt_count() != 0 ||
                     irqs_disabled()) {
                         WARN_ON(1);
@@ -229,7 +230,6 @@ static int rcuhead_fixup_free(void *addr, enum debug_obj_state state)
                 rcu_barrier_bh();
                 debug_object_free(head, &rcuhead_debug_descr);
                 return 1;
-#endif
         default:
                 return 0;
         }
diff --git a/kernel/rcutiny_plugin.h b/kernel/rcutiny_plugin.h
index 015abaea962a..3cb8e362e883 100644
--- a/kernel/rcutiny_plugin.h
+++ b/kernel/rcutiny_plugin.h
@@ -852,7 +852,7 @@ void exit_rcu(void)
         if (t->rcu_read_lock_nesting == 0)
                 return;
         t->rcu_read_lock_nesting = 1;
-        rcu_read_unlock();
+        __rcu_read_unlock();
 }
 
 #else /* #ifdef CONFIG_TINY_PREEMPT_RCU */
diff --git a/kernel/rcutorture.c b/kernel/rcutorture.c
index 89613f97ff26..c224da41890c 100644
--- a/kernel/rcutorture.c
+++ b/kernel/rcutorture.c
@@ -47,7 +47,6 @@
 #include <linux/srcu.h>
 #include <linux/slab.h>
 #include <asm/byteorder.h>
-#include <linux/sched.h>
 
 MODULE_LICENSE("GPL");
 MODULE_AUTHOR("Paul E. McKenney <paulmck@us.ibm.com> and "