sched/core: Add preempt_count invariant check

Ingo requested I keep my debug check for the preempt_count invariant.
Requested-by: NIngo Molnar <mingo@kernel.org>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: NIngo Molnar <mingo@kernel.org>

kernel/sched/core.c

@@ -2514,6 +2514,10 @@ static struct rq *finish_task_switch(struct task_struct *prev)
 	 *
 	 * Also, see FORK_PREEMPT_COUNT.
 	 */
+	if (WARN_ONCE(preempt_count() != 2*PREEMPT_DISABLE_OFFSET,
+		      "corrupted preempt_count: %s/%d/0x%x\n",
+		      current->comm, current->pid, preempt_count()))
+		preempt_count_set(FORK_PREEMPT_COUNT);
 
 	rq->prev_mm = NULL;