Re: [PATCH] leds: trigger: Disable CPU trigger on PREEMPT_RT

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2021-09-27 21:06:50 [+0200], Pavel Machek wrote:
> Hi!
Hi,

> What makes the CPU trigger special with RT? Other triggers will be
> called from interesting places, too... Johanes pointed out other
> problems with that rwlock, and we are getting rid of the rwlock.

There seem to be a few misunderstandings. Let me clear them up for you:
- A non sleeping lock is spinlock_t and rwlock_t as you point out but
  only on !PREEMPT_RT. In context of PREEMPT_RT both of them are
  sleeping locks.

- An interrupt handler on PREEMPT_RT is always threaded. From threaded
  context sleeping is possible and therefore a spinlock_t can be
  acquired by regular request_irq().

- The timer callback (timer_list or hrtimer) is *usually* invoked from
  softirq context which is preemptible on PREEMPT_RT. Therefore sleeping
  is possible in order to acquire a spinlock_t. The exceptional
  callbacks (which are not invoked from softirq thus the term usually
  previously) must not acquire spinlock_t and are specially marked.

- The idle loop (x86 or ARM makes no difference) is always invoked with
  disabled interrupts and so sleeping is a no no on PREEMPT_RT. So a
  spinlock_t can not be acquired in this context.

You mention "interesting places" above. Either this is not the case or
nobody reported it yet.

> Best regards,
> 								Pavel

Sebastian