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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , linux-hams@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org Cc: syzbot+5fd749c74105b0e1b302@syzkaller.appspotmail.com Subject: [PATCH net-next] 6pack: drop redundant locking and refcounting Date: Tue, 23 Sep 2025 14:07:06 +0800 Message-ID: <20250923060706.10232-1-dqfext@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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 Signed-off-by: Qingfang Deng --- 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