sched: Fix the racy usage of thread_group_cputimer() in fastpath_timer_check()

fastpath_timer_check()->thread_group_cputimer() is racy and
unneeded.

It is racy because another thread can clear ->running before
thread_group_cputimer() takes cputimer->lock. In this case
thread_group_cputimer() will set ->running = true again and call
thread_group_cputime(). But since we do not hold tasklist or
siglock, we can race with fork/exit and copy the wrong results
into cputimer->cputime.

It is unneeded because if ->running == true we can just use
the numbers in cputimer->cputime we already have.

Change fastpath_timer_check() to copy cputimer->cputime into
the local variable under cputimer->lock. We do not re-check
->running under cputimer->lock, run_posix_cpu_timers() does
this check later.

Note: we can add more optimizations on top of this change.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20100611180446.GA13025@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/posix-cpu-timers.c

@@ -1287,7 +1287,10 @@ static inline int fastpath_timer_check(struct task_struct *tsk)
 	if (sig->cputimer.running) {
 		struct task_cputime group_sample;
 
-		thread_group_cputimer(tsk, &group_sample);
+		spin_lock(&sig->cputimer.lock);
+		group_sample = sig->cputimer.cputime;
+		spin_unlock(&sig->cputimer.lock);
+
 		if (task_cputime_expired(&group_sample, &sig->cputime_expires))
 			return 1;
 	}