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Commit 50e9fda8 authored by Dmitry Safonov's avatar Dmitry Safonov Committed by Greg Kroah-Hartman
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tty: Don't block on IO when ldisc change is pending 



[ Upstream commit c96cf923a98d1b094df9f0cf97a83e118817e31b ]

There might be situations where tty_ldisc_lock() has blocked, but there
is already IO on tty and it prevents line discipline changes.
It might theoretically turn into dead-lock.

Basically, provide more priority to pending tty_ldisc_lock() than to
servicing reads/writes over tty.

User-visible issue was reported by Mikulas where on pa-risc with
Debian 5 reboot took either 80 seconds, 3 minutes or 3:25 after proper
locking in tty_reopen().

Cc: Jiri Slaby <jslaby@suse.com>
Reported-by: default avatarMikulas Patocka <mpatocka@redhat.com>
Signed-off-by: default avatarDmitry Safonov <dima@arista.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: default avatarSasha Levin <sashal@kernel.org>
parent 01e5d179

drivers/tty/n_hdlc.c

+2 −2
Original line number Diff line number Diff line
@@ -613,7 +613,7 @@ static ssize_t n_hdlc_tty_read(struct tty_struct *tty, struct file *file, 
		}

			

		/* no data */

		if (file->f_flags & O_NONBLOCK) {

		if (tty_io_nonblock(tty, file)) {

			ret = -EAGAIN;

			break;

		}
@@ -680,7 +680,7 @@ static ssize_t n_hdlc_tty_write(struct tty_struct *tty, struct file *file, 
		if (tbuf)

			break;



		if (file->f_flags & O_NONBLOCK) {

		if (tty_io_nonblock(tty, file)) {

			error = -EAGAIN;

			break;

		}

drivers/tty/n_r3964.c

+1 −1
Original line number Diff line number Diff line
@@ -1078,7 +1078,7 @@ static ssize_t r3964_read(struct tty_struct *tty, struct file *file, 
		pMsg = remove_msg(pInfo, pClient);

		if (pMsg == NULL) {

			/* no messages available. */

			if (file->f_flags & O_NONBLOCK) {

			if (tty_io_nonblock(tty, file)) {

				ret = -EAGAIN;

				goto unlock;

			}

drivers/tty/n_tty.c

+4 −4
Original line number Diff line number Diff line
@@ -1702,7 +1702,7 @@ n_tty_receive_buf_common(struct tty_struct *tty, const unsigned char *cp, 


	down_read(&tty->termios_rwsem);



	while (1) {

	do {

		/*

		 * When PARMRK is set, each input char may take up to 3 chars

		 * in the read buf; reduce the buffer space avail by 3x
@@ -1744,7 +1744,7 @@ n_tty_receive_buf_common(struct tty_struct *tty, const unsigned char *cp, 
			fp += n;

		count -= n;

		rcvd += n;

	}

	} while (!test_bit(TTY_LDISC_CHANGING, &tty->flags));



	tty->receive_room = room;
@@ -2211,7 +2211,7 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file, 
					break;

				if (!timeout)

					break;

				if (file->f_flags & O_NONBLOCK) {

				if (tty_io_nonblock(tty, file)) {

					retval = -EAGAIN;

					break;

				}
@@ -2365,7 +2365,7 @@ static ssize_t n_tty_write(struct tty_struct *tty, struct file *file, 
		}

		if (!nr)

			break;

		if (file->f_flags & O_NONBLOCK) {

		if (tty_io_nonblock(tty, file)) {

			retval = -EAGAIN;

			break;

		}

drivers/tty/tty_ldisc.c

+7 −0
Original line number Diff line number Diff line
@@ -336,6 +336,11 @@ int tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout) 
{

	int ret;



	/* Kindly asking blocked readers to release the read side */

	set_bit(TTY_LDISC_CHANGING, &tty->flags);

	wake_up_interruptible_all(&tty->read_wait);

	wake_up_interruptible_all(&tty->write_wait);



	ret = __tty_ldisc_lock(tty, timeout);

	if (!ret)

		return -EBUSY;
@@ -346,6 +351,8 @@ int tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout) 
void tty_ldisc_unlock(struct tty_struct *tty)

{

	clear_bit(TTY_LDISC_HALTED, &tty->flags);

	/* Can be cleared here - ldisc_unlock will wake up writers firstly */

	clear_bit(TTY_LDISC_CHANGING, &tty->flags);

	__tty_ldisc_unlock(tty);

}

include/linux/tty.h

+7 −0
Original line number Diff line number Diff line
@@ -366,6 +366,7 @@ struct tty_file_private { 
#define TTY_NO_WRITE_SPLIT 	17	/* Preserve write boundaries to driver */

#define TTY_HUPPED 		18	/* Post driver->hangup() */

#define TTY_HUPPING		19	/* Hangup in progress */

#define TTY_LDISC_CHANGING	20	/* Change pending - non-block IO */

#define TTY_LDISC_HALTED	22	/* Line discipline is halted */



/* Values for tty->flow_change */
@@ -383,6 +384,12 @@ static inline void tty_set_flow_change(struct tty_struct *tty, int val) 
	smp_mb();

}



static inline bool tty_io_nonblock(struct tty_struct *tty, struct file *file)

{

	return file->f_flags & O_NONBLOCK ||

		test_bit(TTY_LDISC_CHANGING, &tty->flags);

}



static inline bool tty_io_error(struct tty_struct *tty)

{

	return test_bit(TTY_IO_ERROR, &tty->flags);