[PATCH net-next v2] 6pack: drop redundant locking and refcounting

[Qingfang Deng <dqfext@gmail.com>]

The TTY layer already serializes line discipline operations with
tty->ldisc_sem, so the extra disc_data_lock and refcnt in 6pack
are unnecessary.

Removing them simplifies the code and also resolves a lockdep warning
reported by syzbot. The warning did not indicate a real deadlock, since
the write-side lock was only taken in process context with hardirqs
disabled.

Reported-by: syzbot+5fd749c74105b0e1b302@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/68c858b0.050a0220.3c6139.0d1c.GAE@google.com/
Signed-off-by: Qingfang Deng <dqfext@gmail.com>
---
v2: add Closes tag
 - https://lore.kernel.org/netdev/20250923060706.10232-1-dqfext@gmail.com/

 drivers/net/hamradio/6pack.c | 57 ++++--------------------------------
 1 file changed, 5 insertions(+), 52 deletions(-)

diff --git a/drivers/net/hamradio/6pack.c b/drivers/net/hamradio/6pack.c
index c5e5423e1863..885992951e8a 100644
--- a/drivers/net/hamradio/6pack.c
+++ b/drivers/net/hamradio/6pack.c
@@ -115,8 +115,6 @@ struct sixpack {
 
 	struct timer_list	tx_t;
 	struct timer_list	resync_t;
-	refcount_t		refcnt;
-	struct completion	dead;
 	spinlock_t		lock;
 };
 
@@ -353,42 +351,13 @@ static void sp_bump(struct sixpack *sp, char cmd)
 
 /* ----------------------------------------------------------------------- */
 
-/*
- * We have a potential race on dereferencing tty->disc_data, because the tty
- * layer provides no locking at all - thus one cpu could be running
- * sixpack_receive_buf while another calls sixpack_close, which zeroes
- * tty->disc_data and frees the memory that sixpack_receive_buf is using.  The
- * best way to fix this is to use a rwlock in the tty struct, but for now we
- * use a single global rwlock for all ttys in ppp line discipline.
- */
-static DEFINE_RWLOCK(disc_data_lock);
-                                                                                
-static struct sixpack *sp_get(struct tty_struct *tty)
-{
-	struct sixpack *sp;
-
-	read_lock(&disc_data_lock);
-	sp = tty->disc_data;
-	if (sp)
-		refcount_inc(&sp->refcnt);
-	read_unlock(&disc_data_lock);
-
-	return sp;
-}
-
-static void sp_put(struct sixpack *sp)
-{
-	if (refcount_dec_and_test(&sp->refcnt))
-		complete(&sp->dead);
-}
-
 /*
  * Called by the TTY driver when there's room for more data.  If we have
  * more packets to send, we send them here.
  */
 static void sixpack_write_wakeup(struct tty_struct *tty)
 {
-	struct sixpack *sp = sp_get(tty);
+	struct sixpack *sp = tty->disc_data;
 	int actual;
 
 	if (!sp)
@@ -400,7 +369,7 @@ static void sixpack_write_wakeup(struct tty_struct *tty)
 		clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
 		sp->tx_enable = 0;
 		netif_wake_queue(sp->dev);
-		goto out;
+		return;
 	}
 
 	if (sp->tx_enable) {
@@ -408,9 +377,6 @@ static void sixpack_write_wakeup(struct tty_struct *tty)
 		sp->xleft -= actual;
 		sp->xhead += actual;
 	}
-
-out:
-	sp_put(sp);
 }
 
 /* ----------------------------------------------------------------------- */
@@ -430,7 +396,7 @@ static void sixpack_receive_buf(struct tty_struct *tty, const u8 *cp,
 	if (!count)
 		return;
 
-	sp = sp_get(tty);
+	sp = tty->disc_data;
 	if (!sp)
 		return;
 
@@ -446,7 +412,6 @@ static void sixpack_receive_buf(struct tty_struct *tty, const u8 *cp,
 	}
 	sixpack_decode(sp, cp, count1);
 
-	sp_put(sp);
 	tty_unthrottle(tty);
 }
 
@@ -561,8 +526,6 @@ static int sixpack_open(struct tty_struct *tty)
 
 	spin_lock_init(&sp->lock);
 	spin_lock_init(&sp->rxlock);
-	refcount_set(&sp->refcnt, 1);
-	init_completion(&sp->dead);
 
 	/* !!! length of the buffers. MTU is IP MTU, not PACLEN!  */
 
@@ -638,19 +601,11 @@ static void sixpack_close(struct tty_struct *tty)
 {
 	struct sixpack *sp;
 
-	write_lock_irq(&disc_data_lock);
 	sp = tty->disc_data;
-	tty->disc_data = NULL;
-	write_unlock_irq(&disc_data_lock);
 	if (!sp)
 		return;
 
-	/*
-	 * We have now ensured that nobody can start using ap from now on, but
-	 * we have to wait for all existing users to finish.
-	 */
-	if (!refcount_dec_and_test(&sp->refcnt))
-		wait_for_completion(&sp->dead);
+	tty->disc_data = NULL;
 
 	/* We must stop the queue to avoid potentially scribbling
 	 * on the free buffers. The sp->dead completion is not sufficient
@@ -673,7 +628,7 @@ static void sixpack_close(struct tty_struct *tty)
 static int sixpack_ioctl(struct tty_struct *tty, unsigned int cmd,
 		unsigned long arg)
 {
-	struct sixpack *sp = sp_get(tty);
+	struct sixpack *sp = tty->disc_data;
 	struct net_device *dev;
 	unsigned int tmp, err;
 
@@ -725,8 +680,6 @@ static int sixpack_ioctl(struct tty_struct *tty, unsigned int cmd,
 		err = tty_mode_ioctl(tty, cmd, arg);
 	}
 
-	sp_put(sp);
-
 	return err;
 }
 
-- 
2.43.0