tty: drop tty_ldisc_ops::refcount

The refcount is checked only in tty_unregister_ldisc and EBUSY returned
if it is nonzero. But none of the tty_unregister_ldisc callers act
anyhow if this (or any other) error is returned. So remove
tty_ldisc_ops::refcount completely and make tty_unregister_ldisc return
'void' in the next patches. That means we assume tty_unregister_ldisc is
not called while the ldisc might be in use. That relies on
try_module_get in get_ldops and module_put in put_ldops.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210505091928.22010-18-jslaby@suse.czSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_tty.c

@@ -2450,7 +2450,7 @@ void n_tty_inherit_ops(struct tty_ldisc_ops *ops)
 {
 	*ops = n_tty_ops;
 	ops->owner = NULL;
-	ops->refcount = ops->flags = 0;
+	ops->flags = 0;
 }
 EXPORT_SYMBOL_GPL(n_tty_inherit_ops);
 

drivers/tty/tty_ldisc.c

@@ -69,7 +69,6 @@ int tty_register_ldisc(struct tty_ldisc_ops *new_ldisc)
 
 	raw_spin_lock_irqsave(&tty_ldiscs_lock, flags);
 	tty_ldiscs[new_ldisc->num] = new_ldisc;
-	new_ldisc->refcount = 0;
 	raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);
 
 	return ret;
@@ -90,16 +89,12 @@ EXPORT_SYMBOL(tty_register_ldisc);
 int tty_unregister_ldisc(struct tty_ldisc_ops *ldisc)
 {
 	unsigned long flags;
-	int ret = 0;
 
 	raw_spin_lock_irqsave(&tty_ldiscs_lock, flags);
-	if (tty_ldiscs[ldisc->num]->refcount)
-		ret = -EBUSY;
-	else
 	tty_ldiscs[ldisc->num] = NULL;
 	raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);
 
-	return ret;
+	return 0;
 }
 EXPORT_SYMBOL(tty_unregister_ldisc);
 
@@ -113,11 +108,9 @@ static struct tty_ldisc_ops *get_ldops(int disc)
 	ldops = tty_ldiscs[disc];
 	if (ldops) {
 		ret = ERR_PTR(-EAGAIN);
-		if (try_module_get(ldops->owner)) {
-			ldops->refcount++;
+		if (try_module_get(ldops->owner))
 			ret = ldops;
 	}
-	}
 	raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);
 	return ret;
 }
@@ -127,7 +120,6 @@ static void put_ldops(struct tty_ldisc_ops *ldops)
 	unsigned long flags;
 
 	raw_spin_lock_irqsave(&tty_ldiscs_lock, flags);
-	ldops->refcount--;
 	module_put(ldops->owner);
 	raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);
 }

include/linux/tty_ldisc.h

@@ -208,8 +208,6 @@ struct tty_ldisc_ops {
 				const char *fp, int count);
 
 	struct  module *owner;
-
-	int refcount;
 };
 
 struct tty_ldisc {