usb: cdc-acm: fix race during wakeup blocking TX traffic

commit 93e1c8a638308980309e009cc40b5a57ef87caf1 upstream.

When the kernel is compiled with preemption enabled, the URB completion
handler can run in parallel with the work responsible for waking up the
tty layer. If the URB handler sets the EVENT_TTY_WAKEUP bit during the
call to tty_port_tty_wakeup() to signal that there is room for additional
input, it will be cleared at the end of this call. As a result, TX traffic
on the upper layer will be blocked.

This can be seen with a kernel configured with CONFIG_PREEMPT, and a fast
modem connected with PPP running over a USB CDC-ACM port.

Use test_and_clear_bit() instead, which ensures that each wakeup requested
by the URB completion code will trigger a call to tty_port_tty_wakeup().

Fixes: 1aba579f3cf5 cdc-acm: handle read pipe errors
Signed-off-by: Romain Izard <romain.izard.pro@gmail.com>
Cc: stable <stable@vger.kernel.org>
Acked-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
index 8ab0195f8d32caf02e1b99a2b00adf82e8394262..f736c88950897f9b2cb35acb37989a471467414d 100644 (file)
--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -570,10 +570,8 @@ static void acm_softint(struct work_struct *work)
                clear_bit(EVENT_RX_STALL, &acm->flags);
        }
 
-       if (test_bit(EVENT_TTY_WAKEUP, &acm->flags)) {
+       if (test_and_clear_bit(EVENT_TTY_WAKEUP, &acm->flags))
                tty_port_tty_wakeup(&acm->port);
-               clear_bit(EVENT_TTY_WAKEUP, &acm->flags);
-       }
 }
 
 /*