Re: [PATCH RT v3 5/5] rcutorture: Avoid problematic critical section nesting on RT

[Sebastian Andrzej Siewior]

On 2019-09-17 09:36:22 [-0500], Scott Wood wrote:
> > On non-RT you can (but should not) use the counter part of the function
> > in random order like:
> > 	local_bh_disable();
> > 	local_irq_disable();
> > 	local_bh_enable();
> > 	local_irq_enable();
> 
> Actually even non-RT will assert if you do local_bh_enable() with IRQs
> disabled -- but the other combinations do work, and are used some places via
> spinlocks.  If they are used via direct calls to preempt_disable() or
> local_irq_disable() (or via raw spinlocks), then that will not go away on RT
> and we'll have a problem.

lockdep_assert_irqs_enabled() is a nop with CONFIG_PROVE_LOCKING=N and
RT breaks either way. 

> > Since you _can_ use it in random order Paul wants to test that the
> > random use of those function does not break RCU in any way. Since they
> > can not be used on RT in random order it has been agreed that we keep
> > the test for !RT but disable it on RT.
> 
> For now, yes.  Long term it would be good to keep track of when
> preemption/irqs would be disabled on RT, even when running a non-RT debug
> kernel, and assert when bad things are done with it (assuming an RT-capable
> arch).  Besides detecting these fairly unusual patterns, it could also
> detect earlier the much more common problem of nesting a non-raw spinlock
> inside a raw spinlock or other RT-atomic context.

you will be surprised but we have patches for that. We need first get
rid of other "false positives" before plugging this in.

> -Scott

Sebastian