From 2aff5e2bc62db43e05c814461a08aff0fc2b7fe5 Mon Sep 17 00:00:00 2001 From: Peter Hurley Date: Wed, 5 Nov 2014 12:13:01 -0500 Subject: [PATCH] tty: Change tty lock order to master->slave When releasing the master pty, the slave pty also needs to be locked to prevent concurrent tty count changes for the slave pty and to ensure that only one parallel master and slave release observe the final close, and proceed to destruct the pty pair. Conversely, when releasing the slave pty, locking the master pty is not necessary (since the master's state can be inferred by the slave tty count). Introduce tty_lock_slave()/tty_unlock_slave() which acquires/releases the tty lock of the slave pty. Remove tty_lock_pair()/tty_unlock_pair(). Dropping the tty_lock is no longer required to re-establish a stable lock order. Reviewed-by: Alan Cox Signed-off-by: Peter Hurley Signed-off-by: Greg Kroah-Hartman --- drivers/tty/tty_io.c | 10 ++++++---- drivers/tty/tty_mutex.c | 32 +++++++++++++------------------- include/linux/tty.h | 7 ++----- 3 files changed, 21 insertions(+), 28 deletions(-) diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c index bd7cde3c56ef..4ecee2856ece 100644 --- a/drivers/tty/tty_io.c +++ b/drivers/tty/tty_io.c @@ -1790,7 +1790,9 @@ int tty_release(struct inode *inode, struct file *filp) if (tty->ops->close) tty->ops->close(tty, filp); - tty_unlock(tty); + /* If tty is pty master, lock the slave pty (stable lock order) */ + tty_lock_slave(o_tty); + /* * Sanity check: if tty->count is going to zero, there shouldn't be * any waiters on tty->read_wait or tty->write_wait. We test the @@ -1804,8 +1806,6 @@ int tty_release(struct inode *inode, struct file *filp) * Thus this test wouldn't be triggered at the time the slave closed, * so we do it now. */ - tty_lock_pair(tty, o_tty); - while (1) { do_sleep = 0; @@ -1879,7 +1879,9 @@ int tty_release(struct inode *inode, struct file *filp) /* check whether both sides are closing ... */ final = !tty->count && !(o_tty && o_tty->count); - tty_unlock_pair(tty, o_tty); + tty_unlock_slave(o_tty); + tty_unlock(tty); + /* At this point, the tty->count == 0 should ensure a dead tty cannot be re-opened by a racing opener */ diff --git a/drivers/tty/tty_mutex.c b/drivers/tty/tty_mutex.c index 2e41abebbcba..f43e995c7a0f 100644 --- a/drivers/tty/tty_mutex.c +++ b/drivers/tty/tty_mutex.c @@ -4,6 +4,11 @@ #include #include +/* + * Nested tty locks are necessary for releasing pty pairs. + * The stable lock order is master pty first, then slave pty. + */ + /* Legacy tty mutex glue */ enum { @@ -45,29 +50,18 @@ void __lockfunc tty_unlock(struct tty_struct *tty) } EXPORT_SYMBOL(tty_unlock); -/* - * Getting the big tty mutex for a pair of ttys with lock ordering - * On a non pty/tty pair tty2 can be NULL which is just fine. - */ -void __lockfunc tty_lock_pair(struct tty_struct *tty, - struct tty_struct *tty2) +void __lockfunc tty_lock_slave(struct tty_struct *tty) { - if (tty < tty2) { - tty_lock(tty); - tty_lock_nested(tty2, TTY_MUTEX_NESTED); - } else { - if (tty2 && tty2 != tty) - tty_lock(tty2); + if (tty && tty != tty->link) { + WARN_ON(!mutex_is_locked(&tty->link->legacy_mutex) || + !tty->driver->type == TTY_DRIVER_TYPE_PTY || + !tty->driver->type == PTY_TYPE_SLAVE); tty_lock_nested(tty, TTY_MUTEX_NESTED); } } -EXPORT_SYMBOL(tty_lock_pair); -void __lockfunc tty_unlock_pair(struct tty_struct *tty, - struct tty_struct *tty2) +void __lockfunc tty_unlock_slave(struct tty_struct *tty) { - tty_unlock(tty); - if (tty2 && tty2 != tty) - tty_unlock(tty2); + if (tty && tty != tty->link) + tty_unlock(tty); } -EXPORT_SYMBOL(tty_unlock_pair); diff --git a/include/linux/tty.h b/include/linux/tty.h index af1a7f3e4e4a..a07b4b415db8 100644 --- a/include/linux/tty.h +++ b/include/linux/tty.h @@ -638,11 +638,8 @@ extern long vt_compat_ioctl(struct tty_struct *tty, /* functions for preparation of BKL removal */ extern void __lockfunc tty_lock(struct tty_struct *tty); extern void __lockfunc tty_unlock(struct tty_struct *tty); -extern void __lockfunc tty_lock_pair(struct tty_struct *tty, - struct tty_struct *tty2); -extern void __lockfunc tty_unlock_pair(struct tty_struct *tty, - struct tty_struct *tty2); - +extern void __lockfunc tty_lock_slave(struct tty_struct *tty); +extern void __lockfunc tty_unlock_slave(struct tty_struct *tty); /* * this shall be called only from where BTM is held (like close) * -- 2.20.1