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staging: quatech_usb2: Potential lost wakeup scenario in TIOCMIWAIT

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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@gmail.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
This commit is contained in:
Kautuk Consul 2011-09-14 08:56:21 +05:30 committed by Greg Kroah-Hartman
parent e228b7426e
commit e8df1674d3
1 changed files with 7 additions and 2 deletions
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9
drivers/staging/quatech_usb2/quatech_usb2.c
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@ -916,9 +916,10 @@ static int qt2_ioctl(struct tty_struct *tty,
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 *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 +945,7 @@ 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 */