rtmutex: Turn the plist into an rb-tree

Turn the pi-chains from plist to rb-tree, in the rt_mutex code,
and provide a proper comparison function for -deadline and
-priority tasks.

This is done mainly because:
 - classical prio field of the plist is just an int, which might
   not be enough for representing a deadline;
 - manipulating such a list would become O(nr_deadline_tasks),
   which might be to much, as the number of -deadline task increases.

Therefore, an rb-tree is used, and tasks are queued in it according
to the following logic:
 - among two -priority (i.e., SCHED_BATCH/OTHER/RR/FIFO) tasks, the
   one with the higher (lower, actually!) prio wins;
 - among a -priority and a -deadline task, the latter always wins;
 - among two -deadline tasks, the one with the earliest deadline
   wins.

Queueing and dequeueing functions are changed accordingly, for both
the list of a task's pi-waiters and the list of tasks blocked on
a pi-lock.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Dario Faggioli <raistlin@linux.it>
Signed-off-by: Juri Lelli <juri.lelli@gmail.com>
Signed-off-again-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1383831828-15501-10-git-send-email-juri.lelli@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/init_task.h

@@ -11,6 +11,7 @@
 #include <linux/user_namespace.h>
 #include <linux/securebits.h>
 #include <linux/seqlock.h>
+#include <linux/rbtree.h>
 #include <net/net_namespace.h>
 #include <linux/sched/rt.h>
 
@@ -154,6 +155,14 @@ extern struct task_group root_task_group;
 
 #define INIT_TASK_COMM "swapper"
 
+#ifdef CONFIG_RT_MUTEXES
+# define INIT_RT_MUTEXES(tsk)						\
+	.pi_waiters = RB_ROOT,						\
+	.pi_waiters_leftmost = NULL,
+#else
+# define INIT_RT_MUTEXES(tsk)
+#endif
+
 /*
  *  INIT_TASK is used to set up the first task table, touch at
  * your own risk!. Base=0, limit=0x1fffff (=2MB)
@@ -221,6 +230,7 @@ extern struct task_group root_task_group;
 	INIT_TRACE_RECURSION						\
 	INIT_TASK_RCU_PREEMPT(tsk)					\
 	INIT_CPUSET_SEQ(tsk)						\
+	INIT_RT_MUTEXES(tsk)						\
 	INIT_VTIME(tsk)							\
 }