linux-kernel - [009/262] staging: quatech_usb2: Potential lost wakeup scenario in TIOCMIWAIT

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Date:	Wed, 09 Nov 2011 13:25:31 -0800
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk, Kautuk Consul <consul.kautuk@...il.com>
Subject: [009/262] staging: quatech_usb2: Potential lost wakeup scenario in TIOCMIWAIT

3.0-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Kautuk Consul <consul.kautuk@...il.com>

commit e8df1674d383d2ecc6efa8d7dba74c03aafdfdd7 upstream.

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@...il.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

---
 drivers/staging/quatech_usb2/quatech_usb2.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

--- a/drivers/staging/quatech_usb2/quatech_usb2.c
+++ b/drivers/staging/quatech_usb2/quatech_usb2.c
@@ -916,9 +916,10 @@ static int qt2_ioctl(struct tty_struct *
 		dbg("%s() port %d, cmd == TIOCMIWAIT enter",
 			__func__, port->number);
 		prev_msr_value = port_extra->shadowMSR  & QT2_SERIAL_MSR_MASK;
+		barrier();
+		__set_current_state(TASK_INTERRUPTIBLE);
 		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 +927,12 @@ static int qt2_ioctl(struct tty_struct *
 			/* 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 +945,7 @@ static int qt2_ioctl(struct tty_struct *
 				(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 */


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