Re: [PATCH] serial: flush ldisc after hangup

[Peter Hurley <peter@hurleysoftware.com>]

Hi Josef,

On 03/01/2016 10:02 AM, Josef Bacik wrote:
> We hit a panic pretty consistently in production that looked like this
> 
> PID: 461061  TASK: ffff880203f8bc00  CPU: 2   COMMAND: "kworker/u8:2"
>  #0 [ffff88015834b940] machine_kexec at ffffffff8103c1c5
>  #1 [ffff88015834b990] crash_kexec at ffffffff810cd448
>  #2 [ffff88015834ba60] oops_end at ffffffff81006478
>  #3 [ffff88015834ba90] no_context at ffffffff818c5262
>  #4 [ffff88015834baf0] __bad_area_nosemaphore at ffffffff818c545a
>  #5 [ffff88015834bb40] bad_area_nosemaphore at ffffffff818c548c
>  #6 [ffff88015834bb50] __do_page_fault at ffffffff81045ad5
>  #7 [ffff88015834bbc0] do_page_fault at ffffffff81045efc
>  #8 [ffff88015834bbd0] page_fault at ffffffff818d6b82
>     [exception RIP: __uart_start+0x1a]
>     RIP: ffffffff8152f30a  RSP: ffff88015834bc80  RFLAGS: 00010046
>     RAX: 0000000000000000  RBX: ffffffff822e9920  RCX: 0000000000000036
>     RDX: 0000000000003636  RSI: 00000000000000fe  RDI: ffffffff822e9920
>     RBP: ffff88015834bca8   R8: 0000000000000000   R9: 00000000ffffffff
>     R10: ffff8802546f0d20  R11: 0000000000000000  R12: ffff880254712400
>     R13: 0000000000000286  R14: 00000000000000fe  R15: ffff880254712400
>     ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
>  #9 [ffff88015834bc80] uart_start at ffffffff8152fbf2

Thanks for the report, but where's the rest of the stack trace?

> It was a NULL pointer dereference, the state->port.tty was NULL so when we go to
> check tty->stopped in uart_tx_stopped() we panic.  Looking at the other CPU's we
> were in the middle of uart_open(), and the core actually had a valid pointer in
> state->port.tty, which points to a race between either close or hangup (the only
> two places that set state->port.tty to NULL) and open.  Close already flushes
> the ldisc but hangup does not, which means we could have some characters in the
> receive buffer in between the hangup and the open, and we end up in this
> situation.

Yeah, the race is that the ldisc should not be attempting i/o to
the driver at all. This problem is fixed in -next already, but in the
tty core rather than in each individual tty driver.

Regards,
Peter Hurley

> Signed-off-by: Josef Bacik <jbacik@fb.com>
> ---
>  drivers/tty/serial/serial_core.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> index b1f54ab..93b7a53 100644
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -1527,6 +1527,7 @@ static void uart_hangup(struct tty_struct *tty)
>  		wake_up_interruptible(&port->delta_msr_wait);
>  	}
>  	mutex_unlock(&port->mutex);
> +	tty_ldisc_flush(tty);
>  }
>  
>  static void uart_port_shutdown(struct tty_port *port)
>