irq_work: Warn if there's still work on cpu_down

If we are in nohz and there's still irq_work to be done when the idle
task is about to go offline, give a nasty warning. Everything should
have been flushed from the CPU_DYING notifier already. Further attempts
to enqueue an irq_work are buggy because irqs are disabled by
__cpu_disable(). The best we can do is to report the issue to the user.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>

kernel/irq_work.c

@@ -109,6 +109,9 @@ bool irq_work_needs_cpu(void)
 	if (llist_empty(this_list))
 		return false;
 
+	/* All work should have been flushed before going offline */
+	WARN_ON_ONCE(cpu_is_offline(smp_processor_id()));
+
 	return true;
 }