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

[Krzysztof Kozlowski]

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