[PATCH net] net: hdlc_ppp: Fix issues when mod_timer is called while timer is running

ppp_cp_event is called directly or indirectly by ppp_rx with "ppp->lock"
held. It may call mod_timer to add a new timer. However, at the same time
ppp_timer may be already running and waiting for "ppp->lock". In this
case, there's no need for ppp_timer to continue running and it can just
exit.

If we let ppp_timer continue running, it may call add_timer. This causes
kernel panic because add_timer can't be called with a timer pending.
This patch fixes this problem.

Cc: Krzysztof Halasa <khc@pm.waw.pl>
Signed-off-by: Xie He <redacted>
---
 drivers/net/wan/hdlc_ppp.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/net/wan/hdlc_ppp.c b/drivers/net/wan/hdlc_ppp.c
index 64f855651336..261b53fc8e04 100644
--- a/drivers/net/wan/hdlc_ppp.c
+++ b/drivers/net/wan/hdlc_ppp.c

@@ -569,6 +569,13 @@ static void ppp_timer(struct timer_list *t)
 	unsigned long flags;
 
 	spin_lock_irqsave(&ppp->lock, flags);
+	/* mod_timer could be called after we entered this function but
+	 * before we got the lock.
+	 */
+	if (timer_pending(&proto->timer)) {
+		spin_unlock_irqrestore(&ppp->lock, flags);
+		return;
+	}
 	switch (proto->state) {
 	case STOPPING:
 	case REQ_SENT:

-- 
2.27.0