Re: [PATCH 1/5] gpio: thunderx: avoid potential deadlock

[Piyush Malgujar]

On Mon, May 02, 2022 at 01:18:49PM +0200, Bartosz Golaszewski wrote:
> On Wed, Apr 27, 2022 at 4:46 PM Piyush Malgujar <pmalgujar@xxxxxxxxxxx> wrote:
> >
> > Using irqsave/irqrestore locking variants to avoid any deadlock.
> >
> 
> I see you'll be resending this anyway so would you mind providing an
> example of a deadlock that is possible with no-irqsave variants?
> Thanks.
> 
> Bart
>
Hi Bartosz,

Thanks for the review.

Please find below the issue scenario:
In the case when HARDIRQ-safe -> HARDIRQ-unsafe lock order is detected
and interrupt occurs, deadlock could occur.

========================================================
WARNING: possible irq lock inversion dependency detected
5.18.0-rc6 #4 Not tainted
--------------------------------------------------------
swapper/3/0 just changed the state of lock:
ffff000110904cd8 (lock_class){-...}-{2:2}, at: handle_fasteoi_ack_irq+0x2c/0x1b0
but this lock took another, HARDIRQ-unsafe lock in the past:
 (&txgpio->lock){+.+.}-{2:2}


and interrupts could create inverse lock ordering between them.


other info that might help us debug this:
 Possible interrupt unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&txgpio->lock);
                               local_irq_disable();
                               lock(lock_class);
                               lock(&txgpio->lock);
  <Interrupt>
    lock(lock_class);

 *** DEADLOCK ***

==========================================================

Thanks,
Piyush
> > Signed-off-by: Piyush Malgujar <pmalgujar@xxxxxxxxxxx>
> > ---