Revert "rcu: Decrease memory-barrier usage based on semi-formal proof"

This reverts commit e59fb3120becfb36b22ddb8bd27d065d3cdca499.

This reversion was due to (extreme) boot-time slowdowns on SPARC seen by
Yinghai Lu and on x86 by Ingo
.
This is a non-trivial reversion due to intervening commits.

Conflicts:

	Documentation/RCU/trace.txt
	kernel/rcutree.c

Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 12 insertions, 5 deletions

diff --git a/Documentation/RCU/trace.txt b/Documentation/RCU/trace.txt
index 8173cec473aa..c078ad48f7a1 100644
--- a/Documentation/RCU/trace.txt
+++ b/Documentation/RCU/trace.txt
@@ -99,11 +99,18 @@ o	"qp" indicates that RCU still expects a quiescent state from
 
 o       "dt" is the current value of the dyntick counter that is incremented
         when entering or leaving dynticks idle state, either by the
-        scheduler or by irq.  This number is even if the CPU is in
-        dyntick idle mode and odd otherwise.  The number after the first
-        "/" is the interrupt nesting depth when in dyntick-idle state,
-        or one greater than the interrupt-nesting depth otherwise.
-        The number after the second "/" is the NMI nesting depth.
+        scheduler or by irq.  The number after the "/" is the interrupt
+        nesting depth when in dyntick-idle state, or one greater than
+        the interrupt-nesting depth otherwise.
+
+        This field is displayed only for CONFIG_NO_HZ kernels.
+
+o       "dn" is the current value of the dyntick counter that is incremented
+        when entering or leaving dynticks idle state via NMI.  If both
+        the "dt" and "dn" values are even, then this CPU is in dynticks
+        idle mode and may be ignored by RCU.  If either of these two
+        counters is odd, then RCU must be alert to the possibility of
+        an RCU read-side critical section running on this CPU.
 
         This field is displayed only for CONFIG_NO_HZ kernels.