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

[Waiman Long]

On 10/7/24 11:33 AM, Peter Zijlstra wrote:
> 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.

OK, I will see what I can do to work around this issue.

Cheers,
Longman