The patch below does not apply to the 3.9-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

>From 4746b6c6efcdc3f5ef84f0bc2c39707c6b4e5e24 Mon Sep 17 00:00:00 2001
From: Johan Hovold <[email protected]>
Date: Sun, 5 May 2013 20:32:33 +0200
Subject: [PATCH] USB: serial: clean up chars_in_buffer

No need to grab disconnect mutex in chars_in_buffer now that no
sub-driver is or should be querying hardware buffers anymore. (They
should use wait_until_sent.)

Signed-off-by: Johan Hovold <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

diff --git a/drivers/usb/serial/usb-serial.c b/drivers/usb/serial/usb-serial.c
index 60caf9c..4753c00 100644
--- a/drivers/usb/serial/usb-serial.c
+++ b/drivers/usb/serial/usb-serial.c
@@ -359,20 +359,13 @@ static int serial_chars_in_buffer(struct tty_struct *tty)
 {
        struct usb_serial_port *port = tty->driver_data;
        struct usb_serial *serial = port->serial;
-       int count = 0;
 
        dev_dbg(tty->dev, "%s\n", __func__);
 
-       mutex_lock(&serial->disc_mutex);
-       /* if the device was unplugged then any remaining characters
-          fell out of the connector ;) */
        if (serial->disconnected)
-               count = 0;
-       else
-               count = serial->type->chars_in_buffer(tty);
-       mutex_unlock(&serial->disc_mutex);
+               return 0;
 
-       return count;
+       return serial->type->chars_in_buffer(tty);
 }
 
 static void serial_wait_until_sent(struct tty_struct *tty, int timeout)

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