rcu: Apply results of code inspection of kernel/rcutree_plugin.h

rcu: Apply results of code inspection of kernel/rcutree_plugin.h

o Drop the calls to cpu_quiet() from the online/offline code.
  These are unnecessary, since force_quiescent_state() will
  clean up, and removing them simplifies the code a bit.

o Add a warning to check that we don't enqueue the same blocked
  task twice onto the ->blocked_tasks[] lists.

o Rework the phase computation in rcu_preempt_note_context_switch()
  to be more readable, as suggested by Josh Triplett.

o Disable irqs to close a race between the scheduling clock
  interrupt and rcu_preempt_note_context_switch() WRT the
  ->rcu_read_unlock_special field.

o Add comments to rnp->lock acquisition and release within
  rcu_read_unlock_special() noting that irqs are already
  disabled.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: akpm@linux-foundation.org
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
LKML-Reference: <12532926201851-git-send-email->
Signed-off-by: Ingo Molnar <mingo@elte.hu>