Re: [RESEND RFC PATCH v1 2/2] serial/8250: Avoid getting lock in RT atomic context

[Leonardo Bras]

On Wed, Jan 17, 2024 at 11:44:55PM +0100, Thomas Gleixner wrote:
> On Tue, Jan 16 2024 at 04:37, Leonardo Bras wrote:
> > With PREEMPT_RT enabled, a spin_lock_irqsave() becomes a possibly sleeping
> > spin_lock(), without preempt_disable() or irq_disable().
> >
> > This allows a task T1 to get preempted or interrupted while holding the
> > port->lock. If the preempting task T2 need the lock, spin_lock() code
> > will schedule T1 back until it finishes using the lock, and then go back to
> > T2.
> >
> > There is an issue if a T1 holding port->lock is interrupted by an
> > IRQ, and this IRQ handler needs to get port->lock for writting (printk):
> > spin_lock() code will try to reschedule the interrupt handler, which is in
> > atomic context, causing a BUG() for trying to reschedule/sleep in atomic
> > context.
> >
> > So for the case (PREEMPT_RT && in_atomic()) try to get the lock, and if it
> > fails proceed anyway, just like it's done in oops_in_progress case.
> 
> That's just blantantly wrong. The locks are really only to be ignored
> for the oops case, but not for regular printk.

I agree, but the alternative was to have a BUG() due to scheduling in 
atomic context. This would only ignore the lock if it was already taken 
anyway.

That being said, I agree it is not the best solution for the issue, and 
just sent this in the RFC in order to get feedback on what could be done.

> 
> I assume that this is not against the latest RT kernel as that should
> not have that problem at all.

I am based on torvalds/linux at master branch, so maybe I am missing some 
RT-specific patches. Which tree do you recommend me testing?

> 
> Thanks,
> 
>         tglx
> 

Thank you!
Leo