linux-kernel - Re: [PATCH v5 12/14] serial: liteuart: add IRQ support for the RX path

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Date:   Tue, 22 Nov 2022 10:54:45 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Jiri Slaby <jirislaby@...nel.org>
Cc:     "Gabriel L. Somlo" <gsomlo@...il.com>,
        linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
        gregkh@...uxfoundation.org, kgugala@...micro.com,
        mholenko@...micro.com, joel@....id.au,
        david.abdurachmanov@...il.com, florent@...oy-digital.fr,
        ilpo.jarvinen@...ux.intel.com
Subject: Re: [PATCH v5 12/14] serial: liteuart: add IRQ support for the RX path

On Tue, Nov 22, 2022 at 8:44 AM Jiri Slaby <jirislaby@...nel.org> wrote:
> On 21. 11. 22, 19:50, Gabriel L. Somlo wrote:
> >>>    static void liteuart_timer(struct timer_list *t)
> >>>    {
> >>>     struct liteuart_port *uart = from_timer(uart, t, timer);
> >>>     struct uart_port *port = &uart->port;
> >>> -   liteuart_rx_chars(port);
> >>> -
> >>> +   liteuart_interrupt(0, port);
> >>
> >> Are you sure spin_lock() is safe from this path? I assume so, but have you
> >> thought about it?
> >
> > I checked and at that point `in_serving_softirq()` is true.
> >
> > *However*, after studying spin_lock() and friends for a while, I'm
> > not quite clear on whether that unequivocally translates
> > to "yes, we're safe" :)
>
> Depends whether some hard irq context is grabbing the port lock too. If
> it does, it will spin forever waiting for this soft irq to finish (never
> happens as it was interrupted).
>
> > As such, I'm inclined to switch to `spin_lock_irqsave()` and
> > `spin_unlock_irqrestore()` even in the interrupt handler, which is
> > explicitly stated to be "safe from any context":
> > https://www.kernel.org/doc/html/v4.15/kernel-hacking/locking.html#cheat-sheet-for-locking
>
>
>
> > The alternative could be to set `TIMER_IRQSAFE` in `timer_setup()`,
> > but no other tty driver seems to be doing that, so I'd be a bit off
> > the beaten path there... :)
>
> Ah, no.
>
> > Please do let me know what you think about this, particularly if you
> > consider going the spin_lock_irqsave-everywhere-just-to-be-safe route
> > overkill... :)
>
> If you are unsure about the other contexts, irqsave/restore is the way
> to go. It can be lifted later, if someone investigates harder.

Inside the interrupt handler, plain spin_lock() should be OK.
Inside the timer handler, I think you need to use spin_lock_irqsave(),
as e.g. liteuart_set_termios() (and lots of code in serial_core.c)
already uses spin_lock_irqsave().
Besides, on non-SMP, spin_lock() doesn't do anything, so you need
to rely on disabling the local interrupt.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds