tty: n_gsm: fix deadlock and link starvation in outgoing data path

The current implementation queues up new control and user packets as needed
and processes this queue down to the ldisc in the same code path.
That means that the upper and the lower layer are hard coupled in the code.
Due to this deadlocks can happen as seen below while transmitting data,
especially during ldisc congestion. Furthermore, the data channels starve
the control channel on high transmission load on the ldisc.

Introduce an additional control channel data queue to prevent timeouts and
link hangups during ldisc congestion. This is being processed before the
user channel data queue in gsm_data_kick(), i.e. with the highest priority.
Put the queue to ldisc data path into a workqueue and trigger it whenever
new data has been put into the transmission queue. Change
gsm_dlci_data_sweep() accordingly to fill up the transmission queue until
TX_THRESH_HI. This solves the locking issue, keeps latency low and provides
good performance on high data load.
Note that now all packets from a DLCI are removed from the internal queue
if the associated DLCI was closed. This ensures that no data is sent by the
introduced write task to an already closed DLCI.

BUG: spinlock recursion on CPU#0, test_v24_loop/124
 lock: serial8250_ports+0x3a8/0x7500, .magic: dead4ead, .owner: test_v24_loop/124, .owner_cpu: 0
CPU: 0 PID: 124 Comm: test_v24_loop Tainted: G           O      5.18.0-rc2 #3
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
Call Trace:
 <IRQ>
 dump_stack_lvl+0x34/0x44
 do_raw_spin_lock+0x76/0xa0
 _raw_spin_lock_irqsave+0x72/0x80
 uart_write_room+0x3b/0xc0
 gsm_data_kick+0x14b/0x240 [n_gsm]
 gsmld_write_wakeup+0x35/0x70 [n_gsm]
 tty_wakeup+0x53/0x60
 tty_port_default_wakeup+0x1b/0x30
 serial8250_tx_chars+0x12f/0x220
 serial8250_handle_irq.part.0+0xfe/0x150
 serial8250_default_handle_irq+0x48/0x80
 serial8250_interrupt+0x56/0xa0
 __handle_irq_event_percpu+0x78/0x1f0
 handle_irq_event+0x34/0x70
 handle_fasteoi_irq+0x90/0x1e0
 __common_interrupt+0x69/0x100
 common_interrupt+0x48/0xc0
 asm_common_interrupt+0x1e/0x40
RIP: 0010:__do_softirq+0x83/0x34e
Code: 2a 0a ff 0f b7 ed c7 44 24 10 0a 00 00 00 48 c7 c7 51 2a 64 82 e8 2d
e2 d5 ff 65 66 c7 05 83 af 1e 7e 00 00 fb b8 ff ff ff ff <49> c7 c2 40 61
80 82 0f bc c5 41 89 c4 41 83 c4 01 0f 84 e6 00 00
RSP: 0018:ffffc90000003f98 EFLAGS: 00000286
RAX: 00000000ffffffff RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff82642a51 RDI: ffffffff825bb5e7
RBP: 0000000000000200 R08: 00000008de3271a8 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000030 R14: 0000000000000000 R15: 0000000000000000
 ? __do_softirq+0x73/0x34e
 irq_exit_rcu+0xb5/0x100
 common_interrupt+0xa4/0xc0
 </IRQ>
 <TASK>
 asm_common_interrupt+0x1e/0x40
RIP: 0010:_raw_spin_unlock_irqrestore+0x2e/0x50
Code: 00 55 48 89 fd 48 83 c7 18 53 48 89 f3 48 8b 74 24 10 e8 85 28 36 ff
48 89 ef e8 cd 58 36 ff 80 e7 02 74 01 fb bf 01 00 00 00 <e8> 3d 97 33 ff
65 8b 05 96 23 2b 7e 85 c0 74 03 5b 5d c3 0f 1f 44
RSP: 0018:ffffc9000020fd08 EFLAGS: 00000202
RAX: 0000000000000000 RBX: 0000000000000246 RCX: 0000000000000000
RDX: 0000000000000004 RSI: ffffffff8257fd74 RDI: 0000000000000001
RBP: ffff8880057de3a0 R08: 00000008de233000 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000100 R14: 0000000000000202 R15: ffff8880057df0b8
 ? _raw_spin_unlock_irqrestore+0x23/0x50
 gsmtty_write+0x65/0x80 [n_gsm]
 n_tty_write+0x33f/0x530
 ? swake_up_all+0xe0/0xe0
 file_tty_write.constprop.0+0x1b1/0x320
 ? n_tty_flush_buffer+0xb0/0xb0
 new_sync_write+0x10c/0x190
 vfs_write+0x282/0x310
 ksys_write+0x68/0xe0
 do_syscall_64+0x3b/0x90
 entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x7f3e5e35c15c
Code: 8b 7c 24 08 89 c5 e8 c5 ff ff ff 89 ef 89 44 24 08 e8 58 bc 02 00 8b
44 24 08 48 83 c4 10 5d c3 48 63 ff b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff
ff 76 10 48 8b 15 fd fc 05 00 f7 d8 64 89 02 48 83
RSP: 002b:00007ffcee77cd18 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007ffcee77cd70 RCX: 00007f3e5e35c15c
RDX: 0000000000000100 RSI: 00007ffcee77cd90 RDI: 0000000000000003
RBP: 0000000000000100 R08: 0000000000000000 R09: 7efefefefefefeff
R10: 00007f3e5e3bddeb R11: 0000000000000246 R12: 00007ffcee77ce8f
R13: 0000000000000001 R14: 000056214404e010 R15: 00007ffcee77cd90
 </TASK>

Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20220701122332.2039-1-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 279 insertions, 128 deletions

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 9a3d7db33394..79869f2b570c 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -5,6 +5,14 @@
  *
  *	* THIS IS A DEVELOPMENT SNAPSHOT IT IS NOT A FINAL RELEASE *
  *
+ * Outgoing path:
+ * tty -> DLCI fifo -> scheduler -> GSM MUX data queue    ---o-> ldisc
+ * control message               -> GSM MUX control queue --´
+ *
+ * Incoming path:
+ * ldisc -> gsm_queue() -o--> tty
+ *                        `-> gsm_control_response()
+ *
  * TO DO:
  *	Mostly done:	ioctls for setting modes/timing
  *	Partly done:	hooks so you can pull off frames to non tty devs
@@ -210,6 +218,9 @@ struct gsm_mux {
 	/* Events on the GSM channel */
 	wait_queue_head_t event;
 
+	/* ldisc send work */
+	struct work_struct tx_work;
+
 	/* Bits for GSM mode decoding */
 
 	/* Framing Layer */
@@ -241,7 +252,8 @@ struct gsm_mux {
 	unsigned int tx_bytes;		/* TX data outstanding */
 #define TX_THRESH_HI		8192
 #define TX_THRESH_LO		2048
-	struct list_head tx_list;	/* Pending data packets */
+	struct list_head tx_ctrl_list;	/* Pending control packets */
+	struct list_head tx_data_list;	/* Pending data packets */
 
 	/* Control messages */
 	struct timer_list kick_timer;	/* Kick TX queuing on timeout */
@@ -371,6 +383,11 @@ static const u8 gsm_fcs8[256] = {
 
 static int gsmld_output(struct gsm_mux *gsm, u8 *data, int len);
 static int gsm_modem_update(struct gsm_dlci *dlci, u8 brk);
+static struct gsm_msg *gsm_data_alloc(struct gsm_mux *gsm, u8 addr, int len,
+								u8 ctrl);
+static int gsm_send_packet(struct gsm_mux *gsm, struct gsm_msg *msg);
+static void gsmld_write_trigger(struct gsm_mux *gsm);
+static void gsmld_write_task(struct work_struct *work);
 
 /**
  *	gsm_fcs_add	-	update FCS
@@ -655,57 +672,73 @@ static int gsm_stuff_frame(const u8 *input, u8 *output, int len)
  *	@cr: command/response bit seen as initiator
  *	@control:  control byte including PF bit
  *
- *	Format up and transmit a control frame. These do not go via the
- *	queueing logic as they should be transmitted ahead of data when
- *	they are needed.
- *
- *	FIXME: Lock versus data TX path
+ *	Format up and transmit a control frame. These should be transmitted
+ *	ahead of data when they are needed.
  */
-
-static void gsm_send(struct gsm_mux *gsm, int addr, int cr, int control)
+static int gsm_send(struct gsm_mux *gsm, int addr, int cr, int control)
 {
-	int len;
-	u8 cbuf[10];
-	u8 ibuf[3];
+	struct gsm_msg *msg;
+	u8 *dp;
 	int ocr;
+	unsigned long flags;
+
+	msg = gsm_data_alloc(gsm, addr, 0, control);
+	if (!msg)
+		return -ENOMEM;
 
 	/* toggle C/R coding if not initiator */
 	ocr = cr ^ (gsm->initiator ? 0 : 1);
 
-	switch (gsm->encoding) {
-	case 0:
-		cbuf[0] = GSM0_SOF;
-		cbuf[1] = (addr << 2) | (ocr << 1) | EA;
-		cbuf[2] = control;
-		cbuf[3] = EA;	/* Length of data = 0 */
-		cbuf[4] = 0xFF - gsm_fcs_add_block(INIT_FCS, cbuf + 1, 3);
-		cbuf[5] = GSM0_SOF;
-		len = 6;
-		break;
-	case 1:
-	case 2:
-		/* Control frame + packing (but not frame stuffing) in mode 1 */
-		ibuf[0] = (addr << 2) | (ocr << 1) | EA;
-		ibuf[1] = control;
-		ibuf[2] = 0xFF - gsm_fcs_add_block(INIT_FCS, ibuf, 2);
-		/* Stuffing may double the size worst case */
-		len = gsm_stuff_frame(ibuf, cbuf + 1, 3);
-		/* Now add the SOF markers */
-		cbuf[0] = GSM1_SOF;
-		cbuf[len + 1] = GSM1_SOF;
-		/* FIXME: we can omit the lead one in many cases */
-		len += 2;
-		break;
-	default:
-		WARN_ON(1);
-		return;
-	}
-	gsmld_output(gsm, cbuf, len);
-	if (!gsm->initiator) {
-		cr = cr & gsm->initiator;
-		control = control & ~PF;
+	msg->data -= 3;
+	dp = msg->data;
+	*dp++ = (addr << 2) | (ocr << 1) | EA;
+	*dp++ = control;
+
+	if (gsm->encoding == 0)
+		*dp++ = EA; /* Length of data = 0 */
+
+	*dp = 0xFF - gsm_fcs_add_block(INIT_FCS, msg->data, dp - msg->data);
+	msg->len = (dp - msg->data) + 1;
+
+	gsm_print_packet("Q->", addr, cr, control, NULL, 0);
+
+	spin_lock_irqsave(&gsm->tx_lock, flags);
+	list_add_tail(&msg->list, &gsm->tx_ctrl_list);
+	gsm->tx_bytes += msg->len;
+	spin_unlock_irqrestore(&gsm->tx_lock, flags);
+	gsmld_write_trigger(gsm);
+
+	return 0;
+}
+
+/**
+ *	gsm_dlci_clear_queues	-	remove outstanding data for a DLCI
+ *	@gsm: mux
+ *	@dlci: clear for this DLCI
+ *
+ *	Clears the data queues for a given DLCI.
+ */
+static void gsm_dlci_clear_queues(struct gsm_mux *gsm, struct gsm_dlci *dlci)
+{
+	struct gsm_msg *msg, *nmsg;
+	int addr = dlci->addr;
+	unsigned long flags;
+
+	/* Clear DLCI write fifo first */
+	spin_lock_irqsave(&dlci->lock, flags);
+	kfifo_reset(&dlci->fifo);
+	spin_unlock_irqrestore(&dlci->lock, flags);
+
+	/* Clear data packets in MUX write queue */
+	spin_lock_irqsave(&gsm->tx_lock, flags);
+	list_for_each_entry_safe(msg, nmsg, &gsm->tx_data_list, list) {
+		if (msg->addr != addr)
+			continue;
+		gsm->tx_bytes -= msg->len;
+		list_del(&msg->list);
+		kfree(msg);
 	}
-	gsm_print_packet("-->", addr, cr, control, NULL, 0);
+	spin_unlock_irqrestore(&gsm->tx_lock, flags);
 }
 
 /**
@@ -768,6 +801,45 @@ static struct gsm_msg *gsm_data_alloc(struct gsm_mux *gsm, u8 addr, int len,
 }
 
 /**
+ *	gsm_send_packet	-	sends a single packet
+ *	@gsm: GSM Mux
+ *	@msg: packet to send
+ *
+ *	The given packet is encoded and sent out. No memory is freed.
+ *	The caller must hold the gsm tx lock.
+ */
+static int gsm_send_packet(struct gsm_mux *gsm, struct gsm_msg *msg)
+{
+	int len, ret;
+
+
+	if (gsm->encoding == 0) {
+		gsm->txframe[0] = GSM0_SOF;
+		memcpy(gsm->txframe + 1, msg->data, msg->len);
+		gsm->txframe[msg->len + 1] = GSM0_SOF;
+		len = msg->len + 2;
+	} else {
+		gsm->txframe[0] = GSM1_SOF;
+		len = gsm_stuff_frame(msg->data, gsm->txframe + 1, msg->len);
+		gsm->txframe[len + 1] = GSM1_SOF;
+		len += 2;
+	}
+
+	if (debug & 4)
+		gsm_hex_dump_bytes(__func__, gsm->txframe, len);
+	gsm_print_packet("-->", msg->addr, gsm->initiator, msg->ctrl, msg->data,
+			 msg->len);
+
+	ret = gsmld_output(gsm, gsm->txframe, len);
+	if (ret <= 0)
+		return ret;
+	/* FIXME: Can eliminate one SOF in many more cases */
+	gsm->tx_bytes -= msg->len;
+
+	return 0;
+}
+
+/**
  *	gsm_is_flow_ctrl_msg	-	checks if flow control message
  *	@msg: message to check
  *
@@ -799,59 +871,81 @@ static bool gsm_is_flow_ctrl_msg(struct gsm_msg *msg)
 }
 
 /**
- *	gsm_data_kick		-	poke the queue
+ *	gsm_data_kick	-	poke the queue
  *	@gsm: GSM Mux
- *	@dlci: DLCI sending the data
  *
  *	The tty device has called us to indicate that room has appeared in
- *	the transmit queue. Ram more data into the pipe if we have any
+ *	the transmit queue. Ram more data into the pipe if we have any.
  *	If we have been flow-stopped by a CMD_FCOFF, then we can only
- *	send messages on DLCI0 until CMD_FCON
- *
- *	FIXME: lock against link layer control transmissions
+ *	send messages on DLCI0 until CMD_FCON. The caller must hold
+ *	the gsm tx lock.
  */
-
-static void gsm_data_kick(struct gsm_mux *gsm, struct gsm_dlci *dlci)
+static int gsm_data_kick(struct gsm_mux *gsm)
 {
 	struct gsm_msg *msg, *nmsg;
-	int len;
+	struct gsm_dlci *dlci;
+	int ret;
 
-	list_for_each_entry_safe(msg, nmsg, &gsm->tx_list, list) {
+	clear_bit(TTY_DO_WRITE_WAKEUP, &gsm->tty->flags);
+
+	/* Serialize control messages and control channel messages first */
+	list_for_each_entry_safe(msg, nmsg, &gsm->tx_ctrl_list, list) {
 		if (gsm->constipated && !gsm_is_flow_ctrl_msg(msg))
+			return -EAGAIN;
+		ret = gsm_send_packet(gsm, msg);
+		switch (ret) {
+		case -ENOSPC:
+			return -ENOSPC;
+		case -ENODEV:
+			/* ldisc not open */
+			gsm->tx_bytes -= msg->len;
+			list_del(&msg->list);
+			kfree(msg);
 			continue;
-		if (gsm->encoding != 0) {
-			gsm->txframe[0] = GSM1_SOF;
-			len = gsm_stuff_frame(msg->data,
-						gsm->txframe + 1, msg->len);
-			gsm->txframe[len + 1] = GSM1_SOF;
-			len += 2;
-		} else {
-			gsm->txframe[0] = GSM0_SOF;
-			memcpy(gsm->txframe + 1 , msg->data, msg->len);
-			gsm->txframe[msg->len + 1] = GSM0_SOF;
-			len = msg->len + 2;
-		}
-
-		if (debug & 4)
-			gsm_hex_dump_bytes(__func__, gsm->txframe, len);
-		if (gsmld_output(gsm, gsm->txframe, len) <= 0)
+		default:
+			if (ret >= 0) {
+				list_del(&msg->list);
+				kfree(msg);
+			}
 			break;
-		/* FIXME: Can eliminate one SOF in many more cases */
-		gsm->tx_bytes -= msg->len;
-
-		list_del(&msg->list);
-		kfree(msg);
+		}
+	}
 
-		if (dlci) {
-			tty_port_tty_wakeup(&dlci->port);
-		} else {
-			int i = 0;
+	if (gsm->constipated)
+		return -EAGAIN;
 
-			for (i = 0; i < NUM_DLCI; i++)
-				if (gsm->dlci[i])
-					tty_port_tty_wakeup(&gsm->dlci[i]->port);
+	/* Serialize other channels */
+	if (list_empty(&gsm->tx_data_list))
+		return 0;
+	list_for_each_entry_safe(msg, nmsg, &gsm->tx_data_list, list) {
+		dlci = gsm->dlci[msg->addr];
+		/* Send only messages for DLCIs with valid state */
+		if (dlci->state != DLCI_OPEN) {
+			gsm->tx_bytes -= msg->len;
+			list_del(&msg->list);
+			kfree(msg);
+			continue;
+		}
+		ret = gsm_send_packet(gsm, msg);
+		switch (ret) {
+		case -ENOSPC:
+			return -ENOSPC;
+		case -ENODEV:
+			/* ldisc not open */
+			gsm->tx_bytes -= msg->len;
+			list_del(&msg->list);
+			kfree(msg);
+			continue;
+		default:
+			if (ret >= 0) {
+				list_del(&msg->list);
+				kfree(msg);
+			}
+			break;
 		}
 	}
+
+	return 1;
 }
 
 /**
@@ -900,9 +994,21 @@ static void __gsm_data_queue(struct gsm_dlci *dlci, struct gsm_msg *msg)
 	msg->data = dp;
 
 	/* Add to the actual output queue */
-	list_add_tail(&msg->list, &gsm->tx_list);
+	switch (msg->ctrl & ~PF) {
+	case UI:
+	case UIH:
+		if (msg->addr > 0) {
+			list_add_tail(&msg->list, &gsm->tx_data_list);
+			break;
+		}
+		fallthrough;
+	default:
+		list_add_tail(&msg->list, &gsm->tx_ctrl_list);
+		break;
+	}
 	gsm->tx_bytes += msg->len;
-	gsm_data_kick(gsm, dlci);
+
+	gsmld_write_trigger(gsm);
 	mod_timer(&gsm->kick_timer, jiffies + 10 * gsm->t1 * HZ / 100);
 }
 
@@ -1129,32 +1235,39 @@ static int gsm_dlci_modem_output(struct gsm_mux *gsm, struct gsm_dlci *dlci,
 
 static int gsm_dlci_data_sweep(struct gsm_mux *gsm)
 {
-	int len, ret = 0;
 	/* Priority ordering: We should do priority with RR of the groups */
-	int i = 1;
-
-	while (i < NUM_DLCI) {
-		struct gsm_dlci *dlci;
+	int i, len, ret = 0;
+	bool sent;
+	struct gsm_dlci *dlci;
 
-		if (gsm->tx_bytes > TX_THRESH_HI)
-			break;
-		dlci = gsm->dlci[i];
-		if (dlci == NULL || dlci->constipated) {
-			i++;
-			continue;
+	while (gsm->tx_bytes < TX_THRESH_HI) {
+		for (sent = false, i = 1; i < NUM_DLCI; i++) {
+			dlci = gsm->dlci[i];
+			/* skip unused or blocked channel */
+			if (!dlci || dlci->constipated)
+				continue;
+			/* skip channels with invalid state */
+			if (dlci->state != DLCI_OPEN)
+				continue;
+			/* count the sent data per adaption */
+			if (dlci->adaption < 3 && !dlci->net)
+				len = gsm_dlci_data_output(gsm, dlci);
+			else
+				len = gsm_dlci_data_output_framed(gsm, dlci);
+			/* on error exit */
+			if (len < 0)
+				return ret;
+			if (len > 0) {
+				ret++;
+				sent = true;
+				/* The lower DLCs can starve the higher DLCs! */
+				break;
+			}
+			/* try next */
 		}
-		if (dlci->adaption < 3 && !dlci->net)
-			len = gsm_dlci_data_output(gsm, dlci);
-		else
-			len = gsm_dlci_data_output_framed(gsm, dlci);
-		if (len < 0)
+		if (!sent)
 			break;
-		/* DLCI empty - try the next */
-		if (len == 0)
-			i++;
-		else
-			ret++;
-	}
+	};
 
 	return ret;
 }
@@ -1402,7 +1515,6 @@ static void gsm_control_message(struct gsm_mux *gsm, unsigned int command,
 						const u8 *data, int clen)
 {
 	u8 buf[1];
-	unsigned long flags;
 
 	switch (command) {
 	case CMD_CLD: {
@@ -1424,9 +1536,7 @@ static void gsm_control_message(struct gsm_mux *gsm, unsigned int command,
 		gsm->constipated = false;
 		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, NULL);
-		spin_unlock_irqrestore(&gsm->tx_lock, flags);
+		gsmld_write_trigger(gsm);
 		break;
 	case CMD_FCOFF:
 		/* Modem wants us to STFU */
@@ -1629,8 +1739,6 @@ static int gsm_control_wait(struct gsm_mux *gsm, struct gsm_control *control)
 
 static void gsm_dlci_close(struct gsm_dlci *dlci)
 {
-	unsigned long flags;
-
 	del_timer(&dlci->t1);
 	if (debug & 8)
 		pr_debug("DLCI %d goes closed.\n", dlci->addr);
@@ -1639,17 +1747,16 @@ static void gsm_dlci_close(struct gsm_dlci *dlci)
 	dlci->constipated = true;
 	if (dlci->addr != 0) {
 		tty_port_tty_hangup(&dlci->port, false);
-		spin_lock_irqsave(&dlci->lock, flags);
-		kfifo_reset(&dlci->fifo);
-		spin_unlock_irqrestore(&dlci->lock, flags);
+		gsm_dlci_clear_queues(dlci->gsm, dlci);
 		/* Ensure that gsmtty_open() can return. */
 		tty_port_set_initialized(&dlci->port, 0);
 		wake_up_interruptible(&dlci->port.open_wait);
 	} else
 		dlci->gsm->dead = true;
-	wake_up(&dlci->gsm->event);
 	/* A DLCI 0 close is a MUX termination so we need to kick that
 	   back to userspace somehow */
+	gsm_dlci_data_kick(dlci);
+	wake_up(&dlci->gsm->event);
 }
 
 /**
@@ -1672,6 +1779,7 @@ static void gsm_dlci_open(struct gsm_dlci *dlci)
 	/* Send current modem state */
 	if (dlci->addr)
 		gsm_modem_update(dlci, 0);
+	gsm_dlci_data_kick(dlci);
 	wake_up(&dlci->gsm->event);
 }
 
@@ -2222,7 +2330,7 @@ static void gsm1_receive(struct gsm_mux *gsm, unsigned char c)
 	} else if ((c & ISO_IEC_646_MASK) == XOFF) {
 		gsm->constipated = false;
 		/* Kick the link in case it is idling */
-		gsm_data_kick(gsm, NULL);
+		gsmld_write_trigger(gsm);
 		return;
 	}
 	if (c == GSM1_SOF) {
@@ -2353,6 +2461,9 @@ static void gsm_cleanup_mux(struct gsm_mux *gsm, bool disc)
 	del_timer_sync(&gsm->kick_timer);
 	del_timer_sync(&gsm->t2_timer);
 
+	/* Finish writing to ldisc */
+	flush_work(&gsm->tx_work);
+
 	/* Free up any link layer users and finally the control channel */
 	if (gsm->has_devices) {
 		gsm_unregister_devices(gsm_tty_driver, gsm->num);
@@ -2364,9 +2475,12 @@ static void gsm_cleanup_mux(struct gsm_mux *gsm, bool disc)
 	mutex_unlock(&gsm->mutex);
 	/* Now wipe the queues */
 	tty_ldisc_flush(gsm->tty);
-	list_for_each_entry_safe(txq, ntxq, &gsm->tx_list, list)
+	list_for_each_entry_safe(txq, ntxq, &gsm->tx_ctrl_list, list)
+		kfree(txq);
+	INIT_LIST_HEAD(&gsm->tx_ctrl_list);
+	list_for_each_entry_safe(txq, ntxq, &gsm->tx_data_list, list)
 		kfree(txq);
-	INIT_LIST_HEAD(&gsm->tx_list);
+	INIT_LIST_HEAD(&gsm->tx_data_list);
 }
 
 /**
@@ -2385,6 +2499,7 @@ static int gsm_activate_mux(struct gsm_mux *gsm)
 
 	timer_setup(&gsm->kick_timer, gsm_kick_timer, 0);
 	timer_setup(&gsm->t2_timer, gsm_control_retransmit, 0);
+	INIT_WORK(&gsm->tx_work, gsmld_write_task);
 	init_waitqueue_head(&gsm->event);
 	spin_lock_init(&gsm->control_lock);
 	spin_lock_init(&gsm->tx_lock);
@@ -2494,7 +2609,8 @@ static struct gsm_mux *gsm_alloc_mux(void)
 	spin_lock_init(&gsm->lock);
 	mutex_init(&gsm->mutex);
 	kref_init(&gsm->ref);
-	INIT_LIST_HEAD(&gsm->tx_list);
+	INIT_LIST_HEAD(&gsm->tx_ctrl_list);
+	INIT_LIST_HEAD(&gsm->tx_data_list);
 
 	gsm->t1 = T1;
 	gsm->t2 = T2;
@@ -2651,6 +2767,47 @@ static int gsmld_output(struct gsm_mux *gsm, u8 *data, int len)
 	return gsm->tty->ops->write(gsm->tty, data, len);
 }
 
+
+/**
+ *	gsmld_write_trigger	-	schedule ldisc write task
+ *	@gsm: our mux
+ */
+static void gsmld_write_trigger(struct gsm_mux *gsm)
+{
+	if (!gsm || !gsm->dlci[0] || gsm->dlci[0]->dead)
+		return;
+	schedule_work(&gsm->tx_work);
+}
+
+
+/**
+ *	gsmld_write_task	-	ldisc write task
+ *	@work: our tx write work
+ *
+ *	Writes out data to the ldisc if possible. We are doing this here to
+ *	avoid dead-locking. This returns if no space or data is left for output.
+ */
+static void gsmld_write_task(struct work_struct *work)
+{
+	struct gsm_mux *gsm = container_of(work, struct gsm_mux, tx_work);
+	unsigned long flags;
+	int i, ret;
+
+	/* All outstanding control channel and control messages and one data
+	 * frame is sent.
+	 */
+	ret = -ENODEV;
+	spin_lock_irqsave(&gsm->tx_lock, flags);
+	if (gsm->tty)
+		ret = gsm_data_kick(gsm);
+	spin_unlock_irqrestore(&gsm->tx_lock, flags);
+
+	if (ret >= 0)
+		for (i = 0; i < NUM_DLCI; i++)
+			if (gsm->dlci[i])
+				tty_port_tty_wakeup(&gsm->dlci[i]->port);
+}
+
 /**
  *	gsmld_attach_gsm	-	mode set up
  *	@tty: our tty structure
@@ -2790,6 +2947,7 @@ static int gsmld_open(struct tty_struct *tty)
 
 	timer_setup(&gsm->kick_timer, gsm_kick_timer, 0);
 	timer_setup(&gsm->t2_timer, gsm_control_retransmit, 0);
+	INIT_WORK(&gsm->tx_work, gsmld_write_task);
 
 	return 0;
 }
@@ -2806,16 +2964,9 @@ static int gsmld_open(struct tty_struct *tty)
 static void gsmld_write_wakeup(struct tty_struct *tty)
 {
 	struct gsm_mux *gsm = tty->disc_data;
-	unsigned long flags;
 
 	/* Queue poll */
-	clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
-	spin_lock_irqsave(&gsm->tx_lock, flags);
-	gsm_data_kick(gsm, NULL);
-	if (gsm->tx_bytes < TX_THRESH_LO) {
-		gsm_dlci_data_sweep(gsm);
-	}
-	spin_unlock_irqrestore(&gsm->tx_lock, flags);
+	gsmld_write_trigger(gsm);
 }
 
 /**