Re: [PATCH] locking/rtmutex: Always use trylock in rt_mutex_trylock()

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Oct 07, 2024 at 11:23:32AM -0400, Waiman Long wrote:

> > Is the problem that:
> > 
> > 	sched_tick()
          raw_spin_lock(&rq->__lock);
> > 	  task_tick_mm_cid()
> > 	    task_work_add()
> > 	      kasan_save_stack()
> > 	        idiotic crap while holding rq->__lock ?
> > 
> > Because afaict that is completely insane. And has nothing to do with
> > rtmutex.
> > 
> > We are not going to change rtmutex because instrumentation shit is shit.
> 
> Yes, it is because of KASAN that causes page allocation while holding the
> rq->__lock. Maybe we can blame KASAN for this. It is actually not a problem
> for non-PREEMPT_RT kernel because only trylock is being used. However, we
> don't use trylock all the way when rt_spin_trylock() is being used with
> PREEMPT_RT Kernel. 

It has nothing to do with trylock, an everything to do with scheduler
locks being special.

But even so, trying to squirrel a spinlock inside a raw_spinlock is
dodgy at the best of times, yes it mostly works, but should be avoided
whenever possible.

And instrumentation just doesn't count.

> This is certainly a problem that we need to fix as there
> may be other similar case not involving rq->__lock lurking somewhere.

There cannot be, lock order is:

  rtmutex->wait_lock
    task->pi_lock
      rq->__lock

Trying to subvert that order gets you a splat, any other:

  raw_spin_lock(&foo);
  spin_trylock(&bar);

will 'work', despite probably not being a very good idea.

Any case involving the scheduler locks needs to be eradicated, not
worked around.