lockdep splat ("possible circular locking dependency detected") with PL011 on 5.8

[Will Deacon]

On Tue, Aug 11, 2020 at 12:17:13PM +0100, Will Deacon wrote:
> On Tue, Aug 11, 2020 at 12:38:41PM +0200, peterz at infradead.org wrote:
> > diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
> > index 8efd7c2a34fe..1717790ece2b 100644
> > --- a/drivers/tty/serial/amba-pl011.c
> > +++ b/drivers/tty/serial/amba-pl011.c
> > @@ -308,8 +308,9 @@ static void pl011_write(unsigned int val, const struct uart_amba_port *uap,
> >   */
> >  static int pl011_fifo_to_tty(struct uart_amba_port *uap)
> >  {
> > -	u16 status;
> >  	unsigned int ch, flag, fifotaken;
> > +	int sysrq;
> > +	u16 status;
> >  
> >  	for (fifotaken = 0; fifotaken != 256; fifotaken++) {
> >  		status = pl011_read(uap, REG_FR);
> > @@ -344,10 +345,12 @@ static int pl011_fifo_to_tty(struct uart_amba_port *uap)
> >  				flag = TTY_FRAME;
> >  		}
> >  
> > -		if (uart_handle_sysrq_char(&uap->port, ch & 255))
> > -			continue;
> > +		spin_unlock(&uap->port.lock);
> > +		sysrq = uart_handle_sysrq_char(&uap->port, ch & 255);
> > +		spin_lock(&uap->port.lock);
> >  
> > -		uart_insert_char(&uap->port, ch, UART011_DR_OE, ch, flag);
> > +		if (!sysrq)
> > +			uart_insert_char(&uap->port, ch, UART011_DR_OE, ch, flag);
> >  	}
> >  
> >  	return fifotaken;
> 
> Cheers, that seems to do the trick:
> 
> Tested-by: Will Deacon <will at kernel.org>
> 
> but what I don't understand is why I haven't run into this before, and why
> nobody else seems to be reporting it!
> 
> I'll try some older kernels to see if it ever worked.

I tried 5.4, 4.19, 4.14 and 4.9:

5.4 and 4.19 have the lockdep splat

4.14 doesn't have the splat, yet I don't see why (the driver code hasn't really
changed)

4.9 magic sysrq doesn't work at all (no splat either though)

Will