Commit eb5adc70 authored by Duoming Zhou's avatar Duoming Zhou Committed by Max Filippov
Browse files

arch: xtensa: platforms: Fix deadlock in rs_close() 



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 deletes the redundant timer_lock in order to
prevent the deadlock. Because there is no race condition
between rs_close, rs_open and rs_poll.

Signed-off-by: default avatarDuoming Zhou <duoming@zju.edu.cn>
Message-Id: <20220407154430.22387-1-duoming@zju.edu.cn>
Signed-off-by: default avatarMax Filippov <jcmvbkbc@gmail.com>
parent ee69d4be

arch/xtensa/platforms/iss/console.c

+0 −8
Original line number Diff line number Diff line
@@ -36,24 +36,19 @@ static void rs_poll(struct timer_list *); 
static struct tty_driver *serial_driver;

static struct tty_port serial_port;

static DEFINE_TIMER(serial_timer, rs_poll);

static DEFINE_SPINLOCK(timer_lock);



static int rs_open(struct tty_struct *tty, struct file * filp)

{

	spin_lock_bh(&timer_lock);

	if (tty->count == 1)

		mod_timer(&serial_timer, jiffies + SERIAL_TIMER_VALUE);

	spin_unlock_bh(&timer_lock);



	return 0;

}



static void rs_close(struct tty_struct *tty, struct file * filp)

{

	spin_lock_bh(&timer_lock);

	if (tty->count == 1)

		del_timer_sync(&serial_timer);

	spin_unlock_bh(&timer_lock);

}
@@ -73,8 +68,6 @@ static void rs_poll(struct timer_list *unused) 
	int rd = 1;

	unsigned char c;



	spin_lock(&timer_lock);



	while (simc_poll(0)) {

		rd = simc_read(0, &c, 1);

		if (rd <= 0)
@@ -87,7 +80,6 @@ static void rs_poll(struct timer_list *unused) 
		tty_flip_buffer_push(port);

	if (rd)

		mod_timer(&serial_timer, jiffies + SERIAL_TIMER_VALUE);

	spin_unlock(&timer_lock);

}