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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Tomas Melin <tomas.melin@vaisala.com>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 4.14 319/323] serial: 8250: fix potential deadlock in rs485-mode
Date:   Thu, 20 May 2021 11:23:31 +0200
Message-Id: <20210520092131.177575205@linuxfoundation.org>
In-Reply-To: <20210520092120.115153432@linuxfoundation.org>
References: <20210520092120.115153432@linuxfoundation.org>
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From: Tomas Melin <tomas.melin@vaisala.com>

[ Upstream commit b86f86e8e7c5264bb8f5835d60f9ec840d9f5a7a ]

Canceling hrtimer when holding uart spinlock can deadlock.

CPU0: syscall write
          -> get uart port spinlock
              -> write uart
                  -> start_tx_rs485
                      -> hrtimer_cancel
                          -> wait for hrtimer callback to finish

CPU1: hrtimer IRQ
          -> run hrtimer
              -> em485_handle_stop_tx
                  -> get uart port spinlock

CPU0 is waiting for the hrtimer callback to finish, but the hrtimer
callback running on CPU1 is waiting to get the uart port spinlock.

This deadlock can be avoided by not canceling the hrtimers in these paths.
Setting active_timer=NULL can be done without accessing hrtimer,
and that will effectively cancel operations that would otherwise have been
performed by the hrtimer callback.

Signed-off-by: Tomas Melin <tomas.melin@vaisala.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/tty/serial/8250/8250_port.c |    3 ---
 1 file changed, 3 deletions(-)

--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -1527,7 +1527,6 @@ static inline void __stop_tx(struct uart
 			return;
 
 		em485->active_timer = NULL;
-		hrtimer_cancel(&em485->start_tx_timer);
 
 		__stop_tx_rs485(p);
 	}
@@ -1591,8 +1590,6 @@ static inline void start_tx_rs485(struct
 		serial8250_stop_rx(&up->port);
 
 	em485->active_timer = NULL;
-	if (hrtimer_is_queued(&em485->stop_tx_timer))
-		hrtimer_cancel(&em485->stop_tx_timer);
 
 	mcr = serial8250_in_MCR(up);
 	if (!!(up->port.rs485.flags & SER_RS485_RTS_ON_SEND) !=