Re: [PATCH v2 1/4] serial: Rename vars in uart_get_rs485_config()

[Ilpo Järvinen]

On Tue, 30 Aug 2022, Greg Kroah-Hartman wrote:

> On Tue, Aug 30, 2022 at 10:29:53AM +0300, Ilpo Järvinen wrote:
> > Make variable names to match uart_set_rs485_config() ones:
> > 	- rs485 -> rs485_user
> > 	- aux -> rs485
> > 
> > Reviewed-by: Andy Shevchenko <andy.shevchenko@xxxxxxxxx>
> > Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
> > ---
> >  drivers/tty/serial/serial_core.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> > index 12c87cd201a7..8834414a0b2f 100644
> > --- a/drivers/tty/serial/serial_core.c
> > +++ b/drivers/tty/serial/serial_core.c
> > @@ -1408,16 +1408,16 @@ int uart_rs485_config(struct uart_port *port)
> >  EXPORT_SYMBOL_GPL(uart_rs485_config);
> >  
> >  static int uart_get_rs485_config(struct uart_port *port,
> > -			 struct serial_rs485 __user *rs485)
> > +			 struct serial_rs485 __user *rs485_user)
> >  {
> > +	struct serial_rs485 rs485;
> >  	unsigned long flags;
> > -	struct serial_rs485 aux;
> >  
> >  	spin_lock_irqsave(&port->lock, flags);
> > -	aux = port->rs485;
> > +	rs485 = port->rs485;
> >  	spin_unlock_irqrestore(&port->lock, flags);
> 
> I missed this originally, but why does the lock matter here at all?  You
> are just copying all data out of the structure into an on-stack one, why
> the extra step at all?
> 
> As the structure can change instantly after you release the lock, I
> don't see what the lock is protecting.

At least it cannot return inconsistent serial_rs485 because of the lock. 
Probably not an end of the world if the lock wouldn't be taken and a 
concurrent update of port->rs485 would be allowed to mess up a getted 
serial_rs485 as it seems kind of a high-level/userland concurrency issue 
on the tty. Anyway, that seems to be the only reason really.

It's orthogonal to the patch series though.

-- 
 i.