Re: UART/TTY console deadlock

[Andy Shevchenko]

On Fri, Jul 3, 2020 at 1:53 PM Sergey Senozhatsky
<sergey.senozhatsky@xxxxxxxxx> wrote:
>
> On (20/07/02 11:20), Andy Shevchenko wrote:
> >
> > I didn't look into this deeply, but my understanding that this is something for
> > special case when you have several UART ports sharing the IRQ (multi-port card)
> > and IRQ even maybe undesirable b/c it will confuse real IRQ handler. I don't
> > remember details, but AFAIR IRQ handler does a busyloop to service as much as
> > possible and in between it may release the lock (again, multi-port UART cards),
> > that's why we better avoid IRQ event in the first place.
> > But it's my pure speculation.
>
> Hmm. Would different IRQ handlers use different UART port structures? ->irq
> might be the only thing they share. Even if they use the same port, we
> keep IRQs disabled on local CPU, and UART port locked. To unlock the UART
> port during IRQ, the handler (on another CPU) first needs to acquire the
> port->lock, which is locked by serial8250_do_startup().

It appears to be a RT kernel related. Don't know the state of affairs
in RT nowadays, but maybe still relevant.
See the commit 768aec0b5bcc ("serial: 8250: fix shared interrupts
issues with SMP and RT kernels").


-- 
With Best Regards,
Andy Shevchenko