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

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Message-ID: <8dd90e39-087e-984c-c289-dbfa766359a8@kernel.org>
Date:   Tue, 22 Nov 2022 08:44:12 +0100
From:   Jiri Slaby <jirislaby@...nel.org>
To:     "Gabriel L. Somlo" <gsomlo@...il.com>
Cc:     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,
        geert@...ux-m68k.org, ilpo.jarvinen@...ux.intel.com
Subject: Re: [PATCH v5 12/14] serial: liteuart: add IRQ support for the RX
 path

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.

thanks,
-- 
js