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Subject[PATCH] tty: Don't hold ldisc lock in tty_reopen() if ldisc present
Date
Try to get reference for ldisc during tty_reopen().
If ldisc present, we don't need to do tty_ldisc_reinit() and lock the
write side for line discipline semaphore.
Effectively, it optimizes fast-path for tty_reopen(), but more
importantly it won't interrupt ongoing IO on the tty as no ldisc change
is needed.
Fixes user-visible issue when tty_reopen() interrupted login process for
user with a long password, observed and reported by Lukas.

Fixes: c96cf923a98d ("tty: Don't block on IO when ldisc change is pending")
Fixes: 83d817f41070 ("tty: Hold tty_ldisc_lock() during tty_reopen()")
Cc: Jiri Slaby <jslaby@suse.com>
Reported-by: Lukas F. Hartmann <lukas@mntmn.com>
Tested-by: Lukas F. Hartmann <lukas@mntmn.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
---
drivers/tty/tty_io.c | 20 +++++++++++++-------
1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index bfe9ad85b362..23c6fd238422 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1256,7 +1256,8 @@ static void tty_driver_remove_tty(struct tty_driver *driver, struct tty_struct *
static int tty_reopen(struct tty_struct *tty)
{
struct tty_driver *driver = tty->driver;
- int retval;
+ struct tty_ldisc *ld;
+ int retval = 0;

if (driver->type == TTY_DRIVER_TYPE_PTY &&
driver->subtype == PTY_TYPE_MASTER)
@@ -1268,13 +1269,18 @@ static int tty_reopen(struct tty_struct *tty)
if (test_bit(TTY_EXCLUSIVE, &tty->flags) && !capable(CAP_SYS_ADMIN))
return -EBUSY;

- retval = tty_ldisc_lock(tty, 5 * HZ);
- if (retval)
- return retval;
+ ld = tty_ldisc_ref_wait(tty);
+ if (ld) {
+ tty_ldisc_deref(ld);
+ } else {
+ retval = tty_ldisc_lock(tty, 5 * HZ);
+ if (retval)
+ return retval;

- if (!tty->ldisc)
- retval = tty_ldisc_reinit(tty, tty->termios.c_line);
- tty_ldisc_unlock(tty);
+ if (!tty->ldisc)
+ retval = tty_ldisc_reinit(tty, tty->termios.c_line);
+ tty_ldisc_unlock(tty);
+ }

if (retval == 0)
tty->count++;
--
2.20.1
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