Re: [PATCH 06/14] irq: Fix invalid wakeup inirq_wait_for_interrupt

[Thomas Gleixner]

On Thu, 29 Aug 2013, Libin wrote:
> If this thread is preempted at(or before) location a, and the other thread
> set the condition(kthread_stop). After that when this thread is re-scheduled,
> calling set_current_state to set itself as state TASK_INTERRUPTIBLE, if it is
> preempted again after that and before __set_current_state(TASK_RUNNING), it
> triggers the invalid wakeup problem.
> -----------------------
> irq_wait_for_interrupt()
> -----------------------
> ...
> //location a
> set_current_state(TASK_INTERRUPTIBLE);
> //location b
> while (!kthread_should_stop()) {
> 
> 	if (test_and_clear_bit(IRQTF_RUNTHREAD,
> 					   &action->thread_flags)) {
> 			__set_current_state(TASK_RUNNING);
> 			return 0;
> 	}
> 	schedule();//location c
> 	set_current_state(TASK_INTERRUPTIBLE);
> 	//location d
> }
> __set_current_state(TASK_RUNNING);
> ...
> 
> The following circumstance will also trigger this issue:
> At location c, consumer is scheduled out, and be preempted after calling
> set_current_state(TASK_INTERRUPTIBLE) when it be re-schdeuled.
> 
> To solve this problem, using preempt_disable() to bound the operaion that
> setting the task state and the conditions(set by the wake thread) validation.

This is a completely pointless exercise. The irq threads are only
woken from the core code itself and not by some random other threads.

The core code already handles the legit wakeups correctly without
having the need for preemption disable.

Thanks,

	tglx
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