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Date:	Fri,  8 Aug 2014 12:25:52 -0700
From:	Kamal Mostafa <>
To:	Tyler Hall <>
Cc:	Oliver Hartkopp <>,
	Andre Naujoks <>,
	"David S. Miller" <>,, Kamal Mostafa <>,
Subject: [3.13.y.z extended stable] Patch "slip: Fix deadlock in write_wakeup" has been added to staging queue

This is a note to let you know that I have just added a patch titled

    slip: Fix deadlock in write_wakeup

to the linux-3.13.y-queue branch of the 3.13.y.z extended stable tree 
which can be found at:;a=shortlog;h=refs/heads/linux-3.13.y-queue

This patch is scheduled to be released in version

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.13.y.z tree, see



>From ccd4bc19b8d2615e62b8be84874e67bac001f525 Mon Sep 17 00:00:00 2001
From: Tyler Hall <>
Date: Sun, 15 Jun 2014 22:23:16 -0400
Subject: slip: Fix deadlock in write_wakeup

[ Upstream commit 661f7fda21b15ec52f57fcd397c03370acc28688 ]

Use schedule_work() to avoid potentially taking the spinlock in
interrupt context.

Commit cc9fa74e2a ("slip/slcan: added locking in wakeup function") added
necessary locking to the wakeup function and 367525c8c2/ddcde142be ("can:
slcan: Fix spinlock variant") converted it to spin_lock_bh() because the lock
is also taken in timers.

Disabling softirqs is not sufficient, however, as tty drivers may call
write_wakeup from interrupt context. This driver calls tty->ops->write() with
its spinlock held, which may immediately cause an interrupt on the same CPU and
subsequent spin_bug().

Simply converting to spin_lock_irq/irqsave() prevents this deadlock, but
causes lockdep to point out a possible circular locking dependency
between these locks:

(&(&sl->lock)->rlock){-.....}, at: slip_write_wakeup
(&port_lock_key){-.....}, at: serial8250_handle_irq.part.13

The slip transmit is holding the slip spinlock when calling the tty write.
This grabs the port lock. On an interrupt, the handler grabs the port
lock and calls write_wakeup which grabs the slip lock. This could be a
problem if a serial interrupt occurs on another CPU during the slip

To deal with these issues, don't grab the lock in the wakeup function by
deferring the writeout to a workqueue. Also hold the lock during close
when de-assigning the tty pointer to safely disarm the worker and

This bug is easily reproducible on the first transmit when slip is
used with the standard 8250 serial driver.

[<c0410b7c>] (spin_bug+0x0/0x38) from [<c006109c>] (do_raw_spin_lock+0x60/0x1d0)
 r5:eab27000 r4:ec02754c
[<c006103c>] (do_raw_spin_lock+0x0/0x1d0) from [<c04185c0>] (_raw_spin_lock+0x28/0x2c)
 r10:0000001f r9:eabb814c r8:eabb8140 r7:40070193 r6:ec02754c r5:eab27000
 r4:ec02754c r3:00000000
[<c0418598>] (_raw_spin_lock+0x0/0x2c) from [<bf3a0220>] (slip_write_wakeup+0x50/0xe0 [slip])
 r4:ec027540 r3:00000003
[<bf3a01d0>] (slip_write_wakeup+0x0/0xe0 [slip]) from [<c026e420>] (tty_wakeup+0x48/0x68)
 r6:00000000 r5:ea80c480 r4:eab27000 r3:bf3a01d0
[<c026e3d8>] (tty_wakeup+0x0/0x68) from [<c028a8ec>] (uart_write_wakeup+0x2c/0x30)
 r5:ed68ea90 r4:c06790d8
[<c028a8c0>] (uart_write_wakeup+0x0/0x30) from [<c028dc44>] (serial8250_tx_chars+0x114/0x170)
[<c028db30>] (serial8250_tx_chars+0x0/0x170) from [<c028dffc>] (serial8250_handle_irq+0xa0/0xbc)
 r6:000000c2 r5:00000060 r4:c06790d8 r3:00000000
[<c028df5c>] (serial8250_handle_irq+0x0/0xbc) from [<c02933a4>] (dw8250_handle_irq+0x38/0x64)
 r7:00000000 r6:edd2f390 r5:000000c2 r4:c06790d8
[<c029336c>] (dw8250_handle_irq+0x0/0x64) from [<c028d2f4>] (serial8250_interrupt+0x44/0xc4)
 r6:00000000 r5:00000000 r4:c06791c4 r3:c029336c
[<c028d2b0>] (serial8250_interrupt+0x0/0xc4) from [<c0067fe4>] (handle_irq_event_percpu+0xb4/0x2b0)
 r10:c06790d8 r9:eab27000 r8:00000000 r7:00000000 r6:0000001f r5:edd52980
 r4:ec53b6c0 r3:c028d2b0
[<c0067f30>] (handle_irq_event_percpu+0x0/0x2b0) from [<c006822c>] (handle_irq_event+0x4c/0x6c)
 r10:c06790d8 r9:eab27000 r8:c0673ae0 r7:c05c2020 r6:ec53b6c0 r5:edd529d4
[<c00681e0>] (handle_irq_event+0x0/0x6c) from [<c006b140>] (handle_level_irq+0xe8/0x100)
 r6:00000000 r5:edd529d4 r4:edd52980 r3:00022000
[<c006b058>] (handle_level_irq+0x0/0x100) from [<c00676f8>] (generic_handle_irq+0x30/0x40)
 r5:0000001f r4:0000001f
[<c00676c8>] (generic_handle_irq+0x0/0x40) from [<c000f57c>] (handle_IRQ+0xd0/0x13c)
 r4:ea997b18 r3:000000e0
[<c000f4ac>] (handle_IRQ+0x0/0x13c) from [<c00086c4>] (armada_370_xp_handle_irq+0x4c/0x118)
 r8:000003ff r7:ea997b18 r6:ffffffff r5:60070013 r4:c0674dc0
[<c0008678>] (armada_370_xp_handle_irq+0x0/0x118) from [<c0013840>] (__irq_svc+0x40/0x70)
Exception stack(0xea997b18 to 0xea997b60)
7b00:                                                       00000001 20070013
7b20: 00000000 0000000b 20070013 eab27000 20070013 00000000 ed10103e eab27000
7b40: c06790d8 ea997b74 ea997b60 ea997b60 c04186c0 c04186c8 60070013 ffffffff
 r9:eab27000 r8:ed10103e r7:ea997b4c r6:ffffffff r5:60070013 r4:c04186c8
[<c04186a4>] (_raw_spin_unlock_irqrestore+0x0/0x54) from [<c0288fc0>] (uart_start+0x40/0x44)
 r4:c06790d8 r3:c028ddd8
[<c0288f80>] (uart_start+0x0/0x44) from [<c028982c>] (uart_write+0xe4/0xf4)
 r6:0000003e r5:00000000 r4:ed68ea90 r3:0000003e
[<c0289748>] (uart_write+0x0/0xf4) from [<bf3a0d20>] (sl_xmit+0x1c4/0x228 [slip])
 r10:ed388e60 r9:0000003c r8:ffffffdd r7:0000003e r6:ec02754c r5:ea717eb8
[<bf3a0b5c>] (sl_xmit+0x0/0x228 [slip]) from [<c0368d74>] (dev_hard_start_xmit+0x39c/0x6d0)
 r8:eaf163c0 r7:ec027000 r6:ea717eb8 r5:00000000 r4:00000000

Signed-off-by: Tyler Hall <>
Cc: Oliver Hartkopp <>
Cc: Andre Naujoks <>
Cc: David S. Miller <>
Signed-off-by: David S. Miller <>
Signed-off-by: Kamal Mostafa <>
 drivers/net/slip/slip.c | 36 ++++++++++++++++++++++++++----------
 drivers/net/slip/slip.h |  1 +
 2 files changed, 27 insertions(+), 10 deletions(-)

diff --git a/drivers/net/slip/slip.c b/drivers/net/slip/slip.c
index ad4a94e..8752644 100644
--- a/drivers/net/slip/slip.c
+++ b/drivers/net/slip/slip.c
@@ -83,6 +83,7 @@
 #include <linux/delay.h>
 #include <linux/init.h>
 #include <linux/slab.h>
+#include <linux/workqueue.h>
 #include "slip.h"
 #include <linux/ip.h>
@@ -416,36 +417,46 @@ static void sl_encaps(struct slip *sl, unsigned char *icp, int len)

- * Called by the driver when there's room for more data.  If we have
- * more packets to send, we send them here.
- */
-static void slip_write_wakeup(struct tty_struct *tty)
+/* Write out any remaining transmit buffer. Scheduled when tty is writable */
+static void slip_transmit(struct work_struct *work)
+	struct slip *sl = container_of(work, struct slip, tx_work);
 	int actual;
-	struct slip *sl = tty->disc_data;

+	spin_lock_bh(&sl->lock);
 	/* First make sure we're connected. */
-	if (!sl || sl->magic != SLIP_MAGIC || !netif_running(sl->dev))
+	if (!sl->tty || sl->magic != SLIP_MAGIC || !netif_running(sl->dev)) {
+		spin_unlock_bh(&sl->lock);
+	}

-	spin_lock_bh(&sl->lock);
 	if (sl->xleft <= 0)  {
 		/* Now serial buffer is almost free & we can start
 		 * transmission of another packet */
-		clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
+		clear_bit(TTY_DO_WRITE_WAKEUP, &sl->tty->flags);

-	actual = tty->ops->write(tty, sl->xhead, sl->xleft);
+	actual = sl->tty->ops->write(sl->tty, sl->xhead, sl->xleft);
 	sl->xleft -= actual;
 	sl->xhead += actual;

+ * Called by the driver when there's room for more data.
+ * Schedule the transmit.
+ */
+static void slip_write_wakeup(struct tty_struct *tty)
+	struct slip *sl = tty->disc_data;
+	schedule_work(&sl->tx_work);
 static void sl_tx_timeout(struct net_device *dev)
 	struct slip *sl = netdev_priv(dev);
@@ -749,6 +760,7 @@ static struct slip *sl_alloc(dev_t line)
 	sl->magic       = SLIP_MAGIC;
 	sl->dev	      	= dev;
+	INIT_WORK(&sl->tx_work, slip_transmit);
 	sl->mode        = SL_MODE_DEFAULT;
 	/* initialize timer_list struct */
@@ -872,8 +884,12 @@ static void slip_close(struct tty_struct *tty)
 	if (!sl || sl->magic != SLIP_MAGIC || sl->tty != tty)

+	spin_lock_bh(&sl->lock);
 	tty->disc_data = NULL;
 	sl->tty = NULL;
+	spin_unlock_bh(&sl->lock);
+	flush_work(&sl->tx_work);

 	/* VSV = very important to remove timers */
diff --git a/drivers/net/slip/slip.h b/drivers/net/slip/slip.h
index 67673cf..cf32aad 100644
--- a/drivers/net/slip/slip.h
+++ b/drivers/net/slip/slip.h
@@ -53,6 +53,7 @@ struct slip {
   struct tty_struct	*tty;		/* ptr to TTY structure		*/
   struct net_device	*dev;		/* easy for intr handling	*/
   spinlock_t		lock;
+  struct work_struct	tx_work;	/* Flushes transmit buffer	*/

   struct slcompress	*slcomp;	/* for header compression 	*/

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