sched: fix circular dependency of rq->lock and kswadp waitqueue lock

There is a deadlock scenario due to the circular dependency of CPU's
rq->lock and kswapd's waitqueue lock.

(1) when kswapd is woken up, try_to_wake_up() is called with it's
waitqueue lock held. It's previous CPU is offline, so it is woken
up on a different CPU. We try to acquire the offline CPU's rq->lock
in either cpufreq change callback or fixup_busy_time()

(2) At the same time, the offline CPU is coming online and init_idle()
is called from __cpu_up(). init_idle() calls __sched_fork() with
rq->lock held. A debug object allocation in hrtimer_init() called
from __sched_fork() is trying to wakeup the kswapd and attempts to
take the waitqueue lock held in the (1) path.

Task specific initialization is done in __sched_fork() and rq->lock
is not held when it is called for other tasks. The same holds true for
the idle task as well. __sched_fork() for the idle task is called only
when the CPU is not active.

Acquire the rq->lock after calling __sched_fork() in init_idle()
to fix this deadlock.

CRs-Fixed: 965873
Change-Id: Ib8a265835c29861dba571c9b2a6b7e75b5cb43ee
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
[satyap: trivial merge conflicts resolution and omitted changes for QHMP]
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>

kernel/sched/core.c

@@ -7868,10 +7868,11 @@ void init_idle(struct task_struct *idle, int cpu)
 	struct rq *rq = cpu_rq(cpu);
 	unsigned long flags;
 
+	__sched_fork(0, idle);
+
 	raw_spin_lock_irqsave(&idle->pi_lock, flags);
 	raw_spin_lock(&rq->lock);
 
-	__sched_fork(0, idle);
 	idle->state = TASK_RUNNING;
 	idle->se.exec_start = sched_clock();