cputimer: Cure lock inversion

There's a lock inversion between the cputimer->lock and rq->lock;
notably the two callchains involved are:

 update_rlimit_cpu()
   sighand->siglock
   set_process_cpu_timer()
     cpu_timer_sample_group()
       thread_group_cputimer()
         cputimer->lock
         thread_group_cputime()
           task_sched_runtime()
             ->pi_lock
             rq->lock

 scheduler_tick()
   rq->lock
   task_tick_fair()
     update_curr()
       account_group_exec()
         cputimer->lock

Where the first one is enabling a CLOCK_PROCESS_CPUTIME_ID timer, and
the second one is keeping up-to-date.

This problem was introduced by e8abccb ("posix-cpu-timers: Cure
SMP accounting oddities").

Cure the problem by removing the cputimer->lock and rq->lock nesting,
this leaves concurrent enablers doing duplicate work, but the time
wasted should be on the same order otherwise wasted spinning on the
lock and the greater-than assignment filter should ensure we preserve
monotonicity.

Reported-by: Dave Jones <davej@redhat.com>
Reported-by: Simon Kirby <sim@hostway.ca>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: stable@kernel.org
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Link: http://lkml.kernel.org/r/1318928713.21167.4.camel@twins
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/posix-cpu-timers.c

@@ -274,18 +274,19 @@ void thread_group_cputimer(struct task_struct *tsk, struct task_cputime *times)
 	struct task_cputime sum;
 	unsigned long flags;
 
-	spin_lock_irqsave(&cputimer->lock, flags);
 	if (!cputimer->running) {
-		cputimer->running = 1;
 		/*
 		 * The POSIX timer interface allows for absolute time expiry
 		 * values through the TIMER_ABSTIME flag, therefore we have
 		 * to synchronize the timer to the clock every time we start
 		 * it.
 		 */
 		thread_group_cputime(tsk, &sum);
+		spin_lock_irqsave(&cputimer->lock, flags);
+		cputimer->running = 1;
 		update_gt_cputime(&cputimer->cputime, &sum);
-	}
+	} else
+		spin_lock_irqsave(&cputimer->lock, flags);
 	*times = cputimer->cputime;
 	spin_unlock_irqrestore(&cputimer->lock, flags);
 }