Re: [PATCH] tty: serial: samsung_tty: remove spinlock flags in interrupt handlers

From: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
To: Johan Hovold <johan@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Jiri Slaby <jirislaby@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	linux-samsung-soc@vger.kernel.org, linux-serial@vger.kernel.org,
	linux-kernel@vger.kernel.org, marcan@marcan.st, arnd@kernel.org,
	Andy Shevchenko <andy.shevchenko@gmail.com>
Subject: Re: [PATCH] tty: serial: samsung_tty: remove spinlock flags in interrupt handlers
Date: Tue, 16 Mar 2021 11:11:43 +0100	[thread overview]
Message-ID: <816834ba-8dc2-05cd-9c81-a11d65980cfd@canonical.com> (raw)
In-Reply-To: <YFCA5jFLV0Cu9YNe@hovoldconsulting.com>

On 16/03/2021 10:56, Johan Hovold wrote:
> On Tue, Mar 16, 2021 at 10:47:53AM +0100, Krzysztof Kozlowski wrote:
>> On 16/03/2021 10:02, Johan Hovold wrote:
>>> On Mon, Mar 15, 2021 at 07:12:12PM +0100, Krzysztof Kozlowski wrote:
>>>> Since interrupt handler is called with disabled local interrupts, there
>>>> is no need to use the spinlock primitives disabling interrupts as well.
>>>
>>> This isn't generally true due to "threadirqs" and that can lead to
>>> deadlocks if the console code is called from hard irq context.
>>>
>>> Now, this is *not* the case for this particular driver since it doesn't
>>> even bother to take the port lock in console_write(). That should
>>> probably be fixed instead.
>>>
>>> See https://lore.kernel.org/r/X7kviiRwuxvPxC8O@localhost.
>>
>> Thanks for the link, quite interesting! For one type of device we have
>> two interrupts (RX and TX) so I guess it's a valid point/risk. However
>> let me try to understand it more.
>>
>> Assuming we had only one interrupt line, how this interrupt handler with
>> threadirqs could be called from hardirq context?
> 
> No, it's console_write() which can end up being called in hard irq
> context and if that path takes the port lock after the now threaded
> interrupt handler has been preempted you have a deadlock.

Thanks, I understand now. I see three patterns shared by serial drivers:

1. Do not take the lock in console_write() handler,
2. Take the lock like:
if (port->sysrq)
    locked = 0;
else if (oops_in_progress)
    locked = spin_trylock_irqsave(&port->lock, flags);
else
    spin_lock_irqsave(&port->lock, flags)

3. Take the lock like above but preceded with local_irq_save().

It seems the choice of pattern depends which driver was used as a base.

Best regards,
Krzysztof