tty: Fix PPP hang under load

Signed-off-by: Alan Cox <alan@redhat.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tty: Fix PPP hang under load
Signed-off-by: Alan Cox <alan@redhat.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Alan Cox · Linus Torvalds
1 parent d0eafc7db8
Showing 2 changed files with 22 additions and 9 deletions Side-by-side Diff
drivers/char/tty_ldisc.c
include/linux/tty.h
@@ -316,8 +316,7 @@
 {
 	/* wait_event is a macro */
 	wait_event(tty_ldisc_wait, tty_ldisc_try(tty));
-	if (tty->ldisc.refcount == 0)
-		printk(KERN_ERR "tty_ldisc_ref_wait\n");
+	WARN_ON(tty->ldisc.refcount == 0);
 	return &tty->ldisc;
 }
  
  
  
@@ -376,15 +375,17 @@
  *	@tty: terminal to activate ldisc on
  *
  *	Set the TTY_LDISC flag when the line discipline can be called
- *	again. Do necessary wakeups for existing sleepers.
+ *	again. Do necessary wakeups for existing sleepers. Clear the LDISC
+ *	changing flag to indicate any ldisc change is now over.
  *
- *	Note: nobody should set this bit except via this function. Clearing
- *	directly is allowed.
+ *	Note: nobody should set the TTY_LDISC bit except via this function.
+ *	Clearing directly is allowed.
  */
  
 void tty_ldisc_enable(struct tty_struct *tty)
 {
 	set_bit(TTY_LDISC, &tty->flags);
+	clear_bit(TTY_LDISC_CHANGING, &tty->flags);
 	wake_up(&tty_ldisc_wait);
 }
  
  
@@ -496,7 +497,14 @@
 	 *	reference to the line discipline. The TTY_LDISC bit
 	 *	prevents anyone taking a reference once it is clear.
 	 *	We need the lock to avoid racing reference takers.
+	 *
+	 *	We must clear the TTY_LDISC bit here to avoid a livelock
+	 *	with a userspace app continually trying to use the tty in
+	 *	parallel to the change and re-referencing the tty.
 	 */
+	clear_bit(TTY_LDISC, &tty->flags);
+	if (o_tty)
+		clear_bit(TTY_LDISC, &o_tty->flags);
  
 	spin_lock_irqsave(&tty_ldisc_lock, flags);
 	if (tty->ldisc.refcount || (o_tty && o_tty->ldisc.refcount)) {
@@ -528,7 +536,7 @@
 	 *	If the TTY_LDISC bit is set, then we are racing against
 	 *	another ldisc change
 	 */
-	if (!test_bit(TTY_LDISC, &tty->flags)) {
+	if (test_bit(TTY_LDISC_CHANGING, &tty->flags)) {
 		struct tty_ldisc *ld;
 		spin_unlock_irqrestore(&tty_ldisc_lock, flags);
 		tty_ldisc_put(new_ldisc.ops);
  
@@ -536,10 +544,14 @@
 		tty_ldisc_deref(ld);
 		goto restart;
 	}
-
-	clear_bit(TTY_LDISC, &tty->flags);
+	/*
+	 *	This flag is used to avoid two parallel ldisc changes. Once
+	 *	open and close are fine grained locked this may work better
+	 *	as a mutex shared with the open/close/hup paths
+	 */
+	set_bit(TTY_LDISC_CHANGING, &tty->flags);
 	if (o_tty)
-		clear_bit(TTY_LDISC, &o_tty->flags);
+		set_bit(TTY_LDISC_CHANGING, &o_tty->flags);
 	spin_unlock_irqrestore(&tty_ldisc_lock, flags);
  
 	/*
@@ -301,6 +301,7 @@
 #define TTY_PUSH 		6	/* n_tty private */
 #define TTY_CLOSING 		7	/* ->close() in progress */
 #define TTY_LDISC 		9	/* Line discipline attached */
+#define TTY_LDISC_CHANGING 	10	/* Line discipline changing */
 #define TTY_HW_COOK_OUT 	14	/* Hardware can do output cooking */
 #define TTY_HW_COOK_IN 		15	/* Hardware can do input cooking */
 #define TTY_PTY_LOCK 		16	/* pty private */
...	...	@@ -316,8 +316,7 @@
316	316	{
317	317	/* wait_event is a macro */
318	318	wait_event(tty_ldisc_wait, tty_ldisc_try(tty));
319		- if (tty->ldisc.refcount == 0)
320		- printk(KERN_ERR "tty_ldisc_ref_wait\n");
	319	+ WARN_ON(tty->ldisc.refcount == 0);
321	320	return &tty->ldisc;
322	321	}
323	322
324	323
325	324
...	...	@@ -376,15 +375,17 @@
376	375	* @tty: terminal to activate ldisc on
377	376	*
378	377	* Set the TTY_LDISC flag when the line discipline can be called
379		- * again. Do necessary wakeups for existing sleepers.
	378	+ * again. Do necessary wakeups for existing sleepers. Clear the LDISC
	379	+ * changing flag to indicate any ldisc change is now over.
380	380	*
381		- * Note: nobody should set this bit except via this function. Clearing
382		- * directly is allowed.
	381	+ * Note: nobody should set the TTY_LDISC bit except via this function.
	382	+ * Clearing directly is allowed.
383	383	*/
384	384
385	385	void tty_ldisc_enable(struct tty_struct *tty)
386	386	{
387	387	set_bit(TTY_LDISC, &tty->flags);
	388	+ clear_bit(TTY_LDISC_CHANGING, &tty->flags);
388	389	wake_up(&tty_ldisc_wait);
389	390	}
390	391
391	392
...	...	@@ -496,7 +497,14 @@
496	497	* reference to the line discipline. The TTY_LDISC bit
497	498	* prevents anyone taking a reference once it is clear.
498	499	* We need the lock to avoid racing reference takers.
	500	+ *
	501	+ * We must clear the TTY_LDISC bit here to avoid a livelock
	502	+ * with a userspace app continually trying to use the tty in
	503	+ * parallel to the change and re-referencing the tty.
499	504	*/
	505	+ clear_bit(TTY_LDISC, &tty->flags);
	506	+ if (o_tty)
	507	+ clear_bit(TTY_LDISC, &o_tty->flags);
500	508
501	509	spin_lock_irqsave(&tty_ldisc_lock, flags);
502	510	if (tty->ldisc.refcount \|\| (o_tty && o_tty->ldisc.refcount)) {
...	...	@@ -528,7 +536,7 @@
528	536	* If the TTY_LDISC bit is set, then we are racing against
529	537	* another ldisc change
530	538	*/
531		- if (!test_bit(TTY_LDISC, &tty->flags)) {
	539	+ if (test_bit(TTY_LDISC_CHANGING, &tty->flags)) {
532	540	struct tty_ldisc *ld;
533	541	spin_unlock_irqrestore(&tty_ldisc_lock, flags);
534	542	tty_ldisc_put(new_ldisc.ops);
535	543
...	...	@@ -536,10 +544,14 @@
536	544	tty_ldisc_deref(ld);
537	545	goto restart;
538	546	}
539		-
540		- clear_bit(TTY_LDISC, &tty->flags);
	547	+ /*
	548	+ * This flag is used to avoid two parallel ldisc changes. Once
	549	+ * open and close are fine grained locked this may work better
	550	+ * as a mutex shared with the open/close/hup paths
	551	+ */
	552	+ set_bit(TTY_LDISC_CHANGING, &tty->flags);
541	553	if (o_tty)
542		- clear_bit(TTY_LDISC, &o_tty->flags);
	554	+ set_bit(TTY_LDISC_CHANGING, &o_tty->flags);
543	555	spin_unlock_irqrestore(&tty_ldisc_lock, flags);
544	556
545	557	/*
...	...	@@ -301,6 +301,7 @@
301	301	#define TTY_PUSH 6 /* n_tty private */
302	302	#define TTY_CLOSING 7 /* ->close() in progress */
303	303	#define TTY_LDISC 9 /* Line discipline attached */
	304	+#define TTY_LDISC_CHANGING 10 /* Line discipline changing */
304	305	#define TTY_HW_COOK_OUT 14 /* Hardware can do output cooking */
305	306	#define TTY_HW_COOK_IN 15 /* Hardware can do input cooking */
306	307	#define TTY_PTY_LOCK 16 /* pty private */