v2 Documentation and rcutorture changes for 3.9

Hello!

The following are changes to documentation and rcutorture:

1. Export trace_clock_local() in order to allow rcutorture event
tracing to emit the time of the beginning of the RCU read-side critical section at the point where a failure is detected.
2. Reduce rcutorture's read-side tracing to include only failures,
thus cutting the number of events down to something reasonable (normally zero, in fact!).
3. Add atomic_xchg() to the list of atomic operations and memory
barriers, courtesy of Richard Braun.
4. Make rcutorture's shuffler task also shuffle recently added
rcutorture kthreads.

Changes since v1:

o Fixed build problem in #2 located by Randy Dunlap based on
diagnosis from Steven Rostedt.

o Added patch #4.

Thanx, Paul

 b/Documentation/atomic_ops.txt      |    2 +
 b/Documentation/memory-barriers.txt |    1 
 b/include/linux/rcupdate.h          |   13 ++++++--
 b/include/trace/events/rcu.h        |   19 +++++++++---
 b/kernel/rcupdate.c                 |    9 +++--
 b/kernel/rcutorture.c               |   55 ++++++++++++++++++++++++++++--------
 b/kernel/trace/trace_clock.c        |    1 
 b/lib/Kconfig.debug                 |    1 
 8 files changed, 79 insertions(+), 22 deletions(-)