Introduce tty_unregister_ldisc()

It's a bit strange to see tty_register_ldisc() call in modules' exit functions.
...

Documentation/tty.txt +1 -1

@@ -22,7 +22,7 @@ copy of the structure. You must not re-r
 discipline even with the same data or your computer again will be eaten by
 demons.
 
-In order to remove a line discipline call tty_register_ldisc passing NULL.
+In order to remove a line discipline call tty_unregister_ldisc().
 In ancient times this always worked. In modern times the function will
 return -EBUSY if the ldisc is currently in use. Since the ldisc referencing
 code manages the module counts this should not usually be a concern.

diff -uprN linux-tty_register_ldisc_000/drivers/char/tty_io.c linux-tty_register_ldisc_001/drivers/char/tty_io.c

drivers/char/tty_io.c +24 -11

@@ -262,17 +262,10 @@ int tty_register_ldisc(int disc, struct 
 		return -EINVAL;
 
 	spin_lock_irqsave(&tty_ldisc_lock, flags);
-	if (new_ldisc) {
-		tty_ldiscs[disc] = *new_ldisc;
-		tty_ldiscs[disc].num = disc;
-		tty_ldiscs[disc].flags |= LDISC_FLAG_DEFINED;
-		tty_ldiscs[disc].refcount = 0;
-	} else {
-		if(tty_ldiscs[disc].refcount)
-			ret = -EBUSY;
-		else
-			tty_ldiscs[disc].flags &= ~LDISC_FLAG_DEFINED;
-	}
+	tty_ldiscs[disc] = *new_ldisc;
+	tty_ldiscs[disc].num = disc;
+	tty_ldiscs[disc].flags |= LDISC_FLAG_DEFINED;
+	tty_ldiscs[disc].refcount = 0;
 	spin_unlock_irqrestore(&tty_ldisc_lock, flags);
 
 	return ret;

@@ -280,6 +273,26 @@ int tty_register_ldisc(int disc, struct 
 
 EXPORT_SYMBOL(tty_register_ldisc);
 
+int tty_unregister_ldisc(int disc)
+{
+	unsigned long flags;
+	int ret = 0;
+
+	if (disc < N_TTY || disc >= NR_LDISCS)
+		return -EINVAL;
+
+	spin_lock_irqsave(&tty_ldisc_lock, flags);
+	if (tty_ldiscs[disc].refcount)
+		ret = -EBUSY;
+	else
+		tty_ldiscs[disc].flags &= ~LDISC_FLAG_DEFINED;
+	spin_unlock_irqrestore(&tty_ldisc_lock, flags);
+
+	return ret;
+}
+
+EXPORT_SYMBOL(tty_unregister_ldisc);
+
 struct tty_ldisc *tty_ldisc_get(int disc)
 {
 	unsigned long flags;

diff -uprN linux-tty_register_ldisc_000/include/linux/tty.h linux-tty_register_ldisc_001/include/linux/tty.h

include/linux/tty.h +1 -1

@@ -345,6 +345,7 @@ extern int tty_check_change(struct tty_s
 extern void stop_tty(struct tty_struct * tty);
 extern void start_tty(struct tty_struct * tty);
 extern int tty_register_ldisc(int disc, struct tty_ldisc *new_ldisc);
+extern int tty_unregister_ldisc(int disc);
 extern int tty_register_driver(struct tty_driver *driver);
 extern int tty_unregister_driver(struct tty_driver *driver);
 extern void tty_register_device(struct tty_driver *driver, unsigned index, struct device *dev);
-

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An unmodified ldisc driver (externally maintained) will continue to call tty_register_ldisc with NULL, but the new behavior will be to set the ldisc pointer to NULL but have LDISC_FLAG_DEFINED set.

This patch is wrong. Please do not apply.

An unmodified ldisc driver (externally maintained) will continue to call tty_register_ldisc with NULL, but the new behavior will be to set the ldisc pointer to NULL but have LDISC_FLAG_DEFINED set.

If you feel it is absolutely necessary to add this new function for cosmetic reasons, then leave the old behavior in place and make tty_unregister_ldisc a wrapper to tty_register_ldisc that uses a NULL pointer.

This preserves sanity and compatibility.

This patch is wrong. Please do not apply.

An unmodified ldisc driver (externally maintained) will continue to call tty_register_ldisc with NULL, but the new behavior will be to set the ldisc pointer to NULL but have LDISC_FLAG_DEFINED set.

If you feel it is absolutely necessary to add this new function for cosmetic reasons, then leave the old behavior in place and make tty_unregister_ldisc a wrapper to tty_register_ldisc that uses a NULL pointer.

Quoted 4 lines · Paul Fulghum

An unmodified ldisc driver (externally maintained) will continue to call tty_register_ldisc with NULL, but the new behavior will be to set the ldisc pointer to NULL but have LDISC_FLAG_DEFINED set.

Quoted 4 lines

If you feel it is absolutely necessary to add this new function for cosmetic reasons, then leave the old behavior in place and make tty_unregister_ldisc a wrapper to tty_register_ldisc that uses a NULL pointer.

Nah, we don't want to allow staying out of tree folks out of sync. Adding a BUG_ON for ldisk beeing NULL is much more reasoable.