From 70bc126471af30bb115e635512dcf6d86fe6e29a Mon Sep 17 00:00:00 2001 From: Peter Hurley Date: Wed, 6 Mar 2013 08:20:52 -0500 Subject: [PATCH] tty: Add safe tty throttle/unthrottle functions The tty driver can become stuck throttled due to race conditions between throttle and unthrottle, when the decision to throttle or unthrottle is conditional. The following example helps to illustrate the race: CPU 0 | CPU 1 | if (condition A) | | | if (!condition A) | unthrottle() throttle() | | Note the converse is also possible; ie., CPU 0 | CPU 1 | | if (!condition A) | if (condition A) | throttle() | | unthrottle() | Add new throttle/unthrottle functions based on the familiar model of task state and schedule/wake. For example, while (1) { tty_set_flow_change(tty, TTY_THROTTLE_SAFE); if (!condition) break; if (!tty_throttle_safe(tty)) break; } __tty_set_flow_change(tty, 0); In this example, if an unthrottle occurs after the condition is evaluated but before tty_throttle_safe(), then tty_throttle_safe() will return non-zero, looping and forcing the re-evaluation of condition. Reported-by: Vincent Pillet Signed-off-by: Peter Hurley Signed-off-by: Greg Kroah-Hartman --- drivers/tty/tty_ioctl.c | 64 +++++++++++++++++++++++++++++++++++++++++ include/linux/tty.h | 18 ++++++++++++ 2 files changed, 82 insertions(+) diff --git a/drivers/tty/tty_ioctl.c b/drivers/tty/tty_ioctl.c index d58b92cc187c..132d452578bb 100644 --- a/drivers/tty/tty_ioctl.c +++ b/drivers/tty/tty_ioctl.c @@ -106,6 +106,7 @@ void tty_throttle(struct tty_struct *tty) if (!test_and_set_bit(TTY_THROTTLED, &tty->flags) && tty->ops->throttle) tty->ops->throttle(tty); + tty->flow_change = 0; mutex_unlock(&tty->termios_mutex); } EXPORT_SYMBOL(tty_throttle); @@ -129,10 +130,73 @@ void tty_unthrottle(struct tty_struct *tty) if (test_and_clear_bit(TTY_THROTTLED, &tty->flags) && tty->ops->unthrottle) tty->ops->unthrottle(tty); + tty->flow_change = 0; mutex_unlock(&tty->termios_mutex); } EXPORT_SYMBOL(tty_unthrottle); +/** + * tty_throttle_safe - flow control + * @tty: terminal + * + * Similar to tty_throttle() but will only attempt throttle + * if tty->flow_change is TTY_THROTTLE_SAFE. Prevents an accidental + * throttle due to race conditions when throttling is conditional + * on factors evaluated prior to throttling. + * + * Returns 0 if tty is throttled (or was already throttled) + */ + +int tty_throttle_safe(struct tty_struct *tty) +{ + int ret = 0; + + mutex_lock(&tty->termios_mutex); + if (!test_bit(TTY_THROTTLED, &tty->flags)) { + if (tty->flow_change != TTY_THROTTLE_SAFE) + ret = 1; + else { + __set_bit(TTY_THROTTLED, &tty->flags); + if (tty->ops->throttle) + tty->ops->throttle(tty); + } + } + mutex_unlock(&tty->termios_mutex); + + return ret; +} + +/** + * tty_unthrottle_safe - flow control + * @tty: terminal + * + * Similar to tty_unthrottle() but will only attempt unthrottle + * if tty->flow_change is TTY_UNTHROTTLE_SAFE. Prevents an accidental + * unthrottle due to race conditions when unthrottling is conditional + * on factors evaluated prior to unthrottling. + * + * Returns 0 if tty is unthrottled (or was already unthrottled) + */ + +int tty_unthrottle_safe(struct tty_struct *tty) +{ + int ret = 0; + + mutex_lock(&tty->termios_mutex); + if (test_bit(TTY_THROTTLED, &tty->flags)) { + if (tty->flow_change != TTY_UNTHROTTLE_SAFE) + ret = 1; + else { + __clear_bit(TTY_THROTTLED, &tty->flags); + if (tty->ops->unthrottle) + tty->ops->unthrottle(tty); + } + } + mutex_unlock(&tty->termios_mutex); + + return ret; +} + /** * tty_wait_until_sent - wait for I/O to finish * @tty: tty we are waiting for diff --git a/include/linux/tty.h b/include/linux/tty.h index c75d886b0307..189ca80494d1 100644 --- a/include/linux/tty.h +++ b/include/linux/tty.h @@ -258,6 +258,7 @@ struct tty_struct { unsigned char warned:1; unsigned char ctrl_status; /* ctrl_lock */ unsigned int receive_room; /* Bytes free for queue */ + int flow_change; struct tty_struct *link; struct fasync_struct *fasync; @@ -318,6 +319,21 @@ struct tty_file_private { #define TTY_WRITE_FLUSH(tty) tty_write_flush((tty)) +/* Values for tty->flow_change */ +#define TTY_THROTTLE_SAFE 1 +#define TTY_UNTHROTTLE_SAFE 2 + +static inline void __tty_set_flow_change(struct tty_struct *tty, int val) +{ + tty->flow_change = val; +} + +static inline void tty_set_flow_change(struct tty_struct *tty, int val) +{ + tty->flow_change = val; + smp_mb(); +} + #ifdef CONFIG_TTY extern void console_init(void); extern void tty_kref_put(struct tty_struct *tty); @@ -400,6 +416,8 @@ extern int tty_write_room(struct tty_struct *tty); extern void tty_driver_flush_buffer(struct tty_struct *tty); extern void tty_throttle(struct tty_struct *tty); extern void tty_unthrottle(struct tty_struct *tty); +extern int tty_throttle_safe(struct tty_struct *tty); +extern int tty_unthrottle_safe(struct tty_struct *tty); extern int tty_do_resize(struct tty_struct *tty, struct winsize *ws); extern void tty_driver_remove_tty(struct tty_driver *driver, struct tty_struct *tty); -- 2.20.1