[PATCH 1/1] quatech_usb2: Potential lost wakeup scenario in TIOCMIWAIT

[Kautuk Consul]

If the usermode app does an ioctl over this serial device  by
using TIOCMIWAIT, then the code will wait by setting the current
task state to TASK_INTERRUPTIBLE and then calling schedule().
This will be woken up by the qt2_process_modem_status on URB
completion when the port_extra->shadowMSR is set to the new
modem status.

However, this could result in a lost wakeup scenario due to a race 
in the logic in the qt2_ioctl(TIOCMIWAIT) loop and the URB completion
for new modem status in qt2_process_modem_status. 
Due to this, the usermode app's task will continue to sleep despite a 
change in the modem status.

Signed-off-by: Kautuk Consul <consul.kautuk at gmail.com>
---
 drivers/staging/quatech_usb2/quatech_usb2.c |    8 ++++++--
 1 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/staging/quatech_usb2/quatech_usb2.c b/drivers/staging/quatech_usb2/quatech_usb2.c
index ca098ca..af7cba2 100644
--- a/drivers/staging/quatech_usb2/quatech_usb2.c
+++ b/drivers/staging/quatech_usb2/quatech_usb2.c
@@ -915,10 +915,10 @@ static int qt2_ioctl(struct tty_struct *tty,
 	} else if (cmd == TIOCMIWAIT) {
 		dbg("%s() port %d, cmd == TIOCMIWAIT enter",
 			__func__, port->number);
+		set_current_state(TASK_INTERRUPTIBLE);
 		prev_msr_value = port_extra->shadowMSR  & QT2_SERIAL_MSR_MASK;
 		while (1) {
 			add_wait_queue(&port_extra->wait, &wait);
-			set_current_state(TASK_INTERRUPTIBLE);
 			schedule();
 			dbg("%s(): port %d, cmd == TIOCMIWAIT here\n",
 				__func__, port->number);
@@ -926,9 +926,12 @@ static int qt2_ioctl(struct tty_struct *tty,
 			/* see if a signal woke us up */
 			if (signal_pending(current))
 				return -ERESTARTSYS;
+			set_current_state(TASK_INTERRUPTIBLE);
 			msr_value = port_extra->shadowMSR & QT2_SERIAL_MSR_MASK;
-			if (msr_value == prev_msr_value)
+			if (msr_value == prev_msr_value) {
+				__set_current_state(TASK_RUNNING);
 				return -EIO;  /* no change - error */
+			}
 			if ((arg & TIOCM_RNG &&
 				((prev_msr_value & QT2_SERIAL_MSR_RI) ==
 					(msr_value & QT2_SERIAL_MSR_RI))) ||
@@ -941,6 +944,7 @@ static int qt2_ioctl(struct tty_struct *tty,
 				(arg & TIOCM_CTS &&
 				((prev_msr_value & QT2_SERIAL_MSR_CTS) ==
 					(msr_value & QT2_SERIAL_MSR_CTS)))) {
+				__set_current_state(TASK_RUNNING);
 				return 0;
 			}
 		} /* end inifinite while */
-- 
1.7.6