slip: Fix deadlock in write_wakeup

tylerwhall · davem330 · commit 661f7fda21b1 · 2014-06-16T21:29:12.000-07:00
Use schedule_work() to avoid potentially taking the spinlock in interrupt context. Commit cc9fa74 ("slip/slcan: added locking in wakeup function") added necessary locking to the wakeup function and 367525c/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 transmit. 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 timers. 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 r4:edd52980 [<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 2007001 7b20: 00000000 0000000b 2007001 eab27000 2007001 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 r4:ec027000 [<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 <tylerwhall@gmail.com> Cc: Oliver Hartkopp <socketcan@hartkopp.net> Cc: Andre Naujoks <nautsch2@gmail.com> Cc: David S. Miller <davem@davemloft.net> Cc: linux-kernel@vger.kernel.org Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git 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"
 #ifdef CONFIG_INET
 #include <linux/ip.h>
@@ -416,36 +417,46 @@ static void sl_encaps(struct slip *sl, unsigned char *icp, int len)
 #endif
 }
 
-/*
- * 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);
 		return;
+	}
 
-	spin_lock_bh(&sl->lock);
 	if (sl->xleft <= 0)  {
 		/* Now serial buffer is almost free & we can start
 		 * transmission of another packet */
 		sl->dev->stats.tx_packets++;
-		clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
+		clear_bit(TTY_DO_WRITE_WAKEUP, &sl->tty->flags);
 		spin_unlock_bh(&sl->lock);
 		sl_unlock(sl);
 		return;
 	}
 
-	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;
 	spin_unlock_bh(&sl->lock);
 }
 
+/*
+ * 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;
 	spin_lock_init(&sl->lock);
+	INIT_WORK(&sl->tx_work, slip_transmit);
 	sl->mode        = SL_MODE_DEFAULT;
 #ifdef CONFIG_SLIP_SMART
 	/* 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)
 		return;
 
+	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 */
 #ifdef CONFIG_SLIP_SMART
diff --git 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	*/
 
 #ifdef SL_INCLUDE_CSLIP
   struct slcompress	*slcomp;	/* for header compression 	*/
