[ldv-project] [question] Is it possible to remove an active port (without shutdown)?

[Saubhik Mukherjee]

On 6/11/21 3:17 PM, Greg KH wrote:
> On Fri, Jun 11, 2021 at 03:03:42PM +0530, Saubhik Mukherjee wrote:
>> Consider the following events involving drivers/tty/serial/owl-uart.c:
>>
>> Suppose the driver is registered, and the owl_uart_probe() was called.
>> Then uart_startup() can be called in serial core. This calls
>> owl_uart_startup() which registers the interrupt handler owl_uart_irq.
>>
>> Now suppose uart_remove_one_port() in serial core is called. This
>> detaches port from the core. This calls owl_uart_release_port(port).
>> This writes NULL to port->membase after iounmap of port->membase from
>> port->dev.
>>
>> During this point, an interrupt is triggered and the interrupt callback
>> owl_uart_irq() is called (parallel with uart_remove_one_port()). This
>> tries to read port->membase to send or receive chars (with spinlock on
>> port->lock). This introduces a race condition on port->membase.
>>
>> QUESTION: Is it possible to remove an active port (without shutdown)?
> 
> You can remove it, if the driver is set up to do so properly.  Odds are
> the owl-uart code is not written to expect that to ever happen.
> 
> How are you "removing" an active port?  What triggers this action?

Thank you for the reply.

The active port is removed (without shutdown) due to the platform 
callback owl_uart_remove() during de-registration of the platform 
driver. The race condition described is due to the interrupt handler, 
owl_uart_irq(), executing in parallel.