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From:   Duoming Zhou <duoming@zju.edu.cn>
To:     linux-kernel@vger.kernel.org
Cc:     chris@zankel.net, jcmvbkbc@gmail.com, mustafa.ismail@intel.com,
        shiraz.saleem@intel.com, jgg@ziepe.ca, wg@grandegger.com,
        mkl@pengutronix.de, davem@davemloft.net, kuba@kernel.org,
        pabeni@redhat.com, jes@trained-monkey.org,
        gregkh@linuxfoundation.org, jirislaby@kernel.org,
        alexander.deucher@amd.com, linux-xtensa@linux-xtensa.org,
        linux-rdma@vger.kernel.org, linux-can@vger.kernel.org,
        netdev@vger.kernel.org, linux-hippi@sunsite.dk,
        linux-staging@lists.linux.dev, linux-serial@vger.kernel.org,
        linux-usb@vger.kernel.org, Duoming Zhou <duoming@zju.edu.cn>
Subject: [PATCH 11/11] arch: xtensa: platforms: Fix deadlock in rs_close()
Date:   Thu,  7 Apr 2022 14:37:48 +0800
Message-Id: <9ca3ab0b40c875b6019f32f031c68a1ae80dd73a.1649310812.git.duoming@zju.edu.cn>
In-Reply-To: <cover.1649310812.git.duoming@zju.edu.cn>
References: <cover.1649310812.git.duoming@zju.edu.cn>
Precedence: bulk

There is a deadlock in rs_close(), which is shown
below:

   (Thread 1)              |      (Thread 2)
                           | rs_open()
rs_close()                 |  mod_timer()
 spin_lock_bh() //(1)      |  (wait a time)
 ...                       | rs_poll()
 del_timer_sync()          |  spin_lock() //(2)
 (wait timer to stop)      |  ...

We hold timer_lock in position (1) of thread 1 and
use del_timer_sync() to wait timer to stop, but timer handler
also need timer_lock in position (2) of thread 2.
As a result, rs_close() will block forever.

This patch extracts del_timer_sync() from the protection of
spin_lock_bh(), which could let timer handler to obtain
the needed lock.

Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
---
 arch/xtensa/platforms/iss/console.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/xtensa/platforms/iss/console.c b/arch/xtensa/platforms/iss/console.c
index 81d7c7e8f7e..d431b61ae3c 100644
--- a/arch/xtensa/platforms/iss/console.c
+++ b/arch/xtensa/platforms/iss/console.c
@@ -51,8 +51,10 @@ static int rs_open(struct tty_struct *tty, struct file * filp)
 static void rs_close(struct tty_struct *tty, struct file * filp)
 {
 	spin_lock_bh(&timer_lock);
-	if (tty->count == 1)
+	if (tty->count == 1) {
+		spin_unlock_bh(&timer_lock);
 		del_timer_sync(&serial_timer);
+	}
 	spin_unlock_bh(&timer_lock);
 }
 
-- 
2.17.1