[PATCH] drivers/tty: fix BUG_ON when calls serial8250_suspend_port

[Yang Yingliang]

My kthread calls serial8250_suspend_port/serial8250_resume_port when the
system is booting, it triggers a BUG_ON:

BUG: failure at drivers/tty/serial/8250/8250_core.c:1852/serial_unlink_irq_chain()!
Kernel panic - not syncing: BUG!
CPU: 21 PID: 927 Comm: uart_debug_thre Not tainted 4.1.18+ #96
Call trace:
[<ffffffc00008988c>] dump_backtrace+0x0/0x128
[<ffffffc0000899c8>] show_stack+0x14/0x1c
[<ffffffc0005e3d8c>] dump_stack+0x88/0xa8
[<ffffffc0005e2cf0>] panic+0xe4/0x228
[<ffffffc0003c92f4>] univ8250_release_irq+0xc0/0x124
[<ffffffc0003c8474>] serial8250_do_shutdown+0xdc/0x144
[<ffffffc0003c84fc>] serial8250_shutdown+0x20/0x28
[<ffffffc0003c26e4>] uart_suspend_port+0x200/0x234
[<ffffffc0003c715c>] serial8250_suspend_port+0x5c/0xac
[<ffffffc000460a50>] uart_debug+0x24/0x3c
[<ffffffc0000cdf58>] kthread+0xdc/0xf0

I found port->flags is used without lock in tty_port_block_til_ready(). The flags
will be overwritten with old value after it updates in uart_suspend_port().
		CPU A				CPU B
	in uart_suspend_port		in tty_port_block_til_ready
					get port->flags
	clear ASYNCB_INITIALIZED
					set port->flags
The flags still has ASYNCB_INITIALIZED. When uart_suspend_port is called next time,
it trriggers the BUG_ON. So use set_bit/clear_bit to avoid the flags being overwritten.

Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Jiri Slaby <jslaby@xxxxxxx>
Signed-off-by: Yang Yingliang <yangyingliang@xxxxxxxxxx>
---
 drivers/tty/tty_port.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/drivers/tty/tty_port.c b/drivers/tty/tty_port.c
index 40b3183..d57cf70 100644
--- a/drivers/tty/tty_port.c
+++ b/drivers/tty/tty_port.c
@@ -237,7 +237,7 @@ void tty_port_hangup(struct tty_port *port)
 
 	spin_lock_irqsave(&port->lock, flags);
 	port->count = 0;
-	port->flags &= ~ASYNC_NORMAL_ACTIVE;
+	clear_bit(ASYNCB_NORMAL_ACTIVE, &port->flags);
 	tty = port->tty;
 	if (tty)
 		set_bit(TTY_IO_ERROR, &tty->flags);
@@ -376,14 +376,14 @@ int tty_port_block_til_ready(struct tty_port *port,
 	/* if non-blocking mode is set we can pass directly to open unless
 	   the port has just hung up or is in another error state */
 	if (tty->flags & (1 << TTY_IO_ERROR)) {
-		port->flags |= ASYNC_NORMAL_ACTIVE;
+		set_bit(ASYNCB_NORMAL_ACTIVE, &port->flags);
 		return 0;
 	}
 	if (filp->f_flags & O_NONBLOCK) {
 		/* Indicate we are open */
 		if (tty->termios.c_cflag & CBAUD)
 			tty_port_raise_dtr_rts(port);
-		port->flags |= ASYNC_NORMAL_ACTIVE;
+		set_bit(ASYNCB_NORMAL_ACTIVE, &port->flags);
 		return 0;
 	}
 
@@ -443,7 +443,7 @@ int tty_port_block_til_ready(struct tty_port *port,
 		port->count++;
 	port->blocked_open--;
 	if (retval == 0)
-		port->flags |= ASYNC_NORMAL_ACTIVE;
+		set_bit(ASYNCB_NORMAL_ACTIVE, &port->flags);
 	spin_unlock_irqrestore(&port->lock, flags);
 	return retval;
 }
@@ -533,7 +533,8 @@ void tty_port_close_end(struct tty_port *port, struct tty_struct *tty)
 		spin_lock_irqsave(&port->lock, flags);
 		wake_up_interruptible(&port->open_wait);
 	}
-	port->flags &= ~(ASYNC_NORMAL_ACTIVE | ASYNC_CLOSING);
+	clear_bit(ASYNCB_NORMAL_ACTIVE, &port->flags);
+	clear_bit(ASYNCB_CLOSING, &port->flags);
 	wake_up_interruptible(&port->close_wait);
 	spin_unlock_irqrestore(&port->lock, flags);
 }
-- 
2.5.0