n_gsm: added interlocking for gsm_data_lock for certain code paths

There were some locking holes in the management of the MUX's
message queue for 2 code paths:
1) gsmld_write_wakeup
2) receipt of CMD_FCON flow-control message
In both cases gsm_data_kick is called w/o locking so it can collide
with other other instances of gsm_data_kick (pulling messages tx_tail)
or potentially other instances of __gsm_data_queu (adding messages to tx_head)

Changed to take the tx_lock in these 2 cases

Signed-off-by: Russ Gorby <russ.gorby@intel.com>
Tested-by: Yin, Fengwei <fengwei.yin@intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Cc: Riding School <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_gsm.c

@@ -1205,6 +1205,8 @@ static void gsm_control_message(struct gsm_mux *gsm, unsigned int command,
 							u8 *data, int clen)
 {
 	u8 buf[1];
+	unsigned long flags;
+
 	switch (command) {
 	case CMD_CLD: {
 		struct gsm_dlci *dlci = gsm->dlci[0];
@@ -1225,7 +1227,9 @@ static void gsm_control_message(struct gsm_mux *gsm, unsigned int command,
 		gsm->constipated = 0;
 		gsm_control_reply(gsm, CMD_FCON, NULL, 0);
 		/* Kick the link in case it is idling */
+		spin_lock_irqsave(&gsm->tx_lock, flags);
 		gsm_data_kick(gsm);
+		spin_unlock_irqrestore(&gsm->tx_lock, flags);
 		break;
 	case CMD_FCOFF:
 		/* Modem wants us to STFU */
@@ -2392,12 +2396,12 @@ static void gsmld_write_wakeup(struct tty_struct *tty)
 
 	/* Queue poll */
 	clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
+	spin_lock_irqsave(&gsm->tx_lock, flags);
 	gsm_data_kick(gsm);
 	if (gsm->tx_bytes < TX_THRESH_LO) {
-		spin_lock_irqsave(&gsm->tx_lock, flags);
 		gsm_dlci_data_sweep(gsm);
-		spin_unlock_irqrestore(&gsm->tx_lock, flags);
 	}
+	spin_unlock_irqrestore(&gsm->tx_lock, flags);
 }
 
 /**