Without serialization, the flow control state can become inverted
wrt. the actual hardware state. For example,
CPU 0 | CPU 1
stop_tty() |
lock ctrl_lock |
tty->stopped = 1 |
unlock ctrl_lock |
| start_tty()
| lock ctrl_lock
| tty->stopped = 0
| unlock ctrl_lock
| driver->start()
driver->stop() |
In this case, the flow control state now indicates the tty has
been started, but the actual hardware state has actually been stopped.
Introduce tty->flow_lock spinlock to serialize tty flow control changes.
Split out unlocked __start_tty()/__stop_tty() flavors for use by
ioctl(TCXONC) in follow-on patch.
Signed-off-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
---
drivers/tty/tty_io.c | 39 ++++++++++++++++++++++++++++-----------
include/linux/tty.h | 5 ++++-
include/linux/tty_driver.h | 4 ++++
3 files changed, 36 insertions(+), 12 deletions(-)
diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 714320b..b898e29 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -933,18 +933,18 @@ void no_tty(void)
* but not always.
*
* Locking:
- * Uses the tty control lock internally
+ * ctrl_lock
+ * flow_lock
*/
-void stop_tty(struct tty_struct *tty)
+void __stop_tty(struct tty_struct *tty)
{
unsigned long flags;
- spin_lock_irqsave(&tty->ctrl_lock, flags);
- if (tty->stopped) {
- spin_unlock_irqrestore(&tty->ctrl_lock, flags);
+
+ if (tty->stopped)
return;
- }
tty->stopped = 1;
+ spin_lock_irqsave(&tty->ctrl_lock, flags);
if (tty->link && tty->link->packet) {
tty->ctrl_status &= ~TIOCPKT_START;
tty->ctrl_status |= TIOCPKT_STOP;
@@ -955,6 +955,14 @@ void stop_tty(struct tty_struct *tty)
(tty->ops->stop)(tty);
}
+void stop_tty(struct tty_struct *tty)
+{
+ unsigned long flags;
+
+ spin_lock_irqsave(&tty->flow_lock, flags);
+ __stop_tty(tty);
+ spin_unlock_irqrestore(&tty->flow_lock, flags);
+}
EXPORT_SYMBOL(stop_tty);
/**
@@ -968,17 +976,17 @@ EXPORT_SYMBOL(stop_tty);
*
* Locking:
* ctrl_lock
+ * flow_lock
*/
-void start_tty(struct tty_struct *tty)
+void __start_tty(struct tty_struct *tty)
{
unsigned long flags;
- spin_lock_irqsave(&tty->ctrl_lock, flags);
- if (!tty->stopped || tty->flow_stopped) {
- spin_unlock_irqrestore(&tty->ctrl_lock, flags);
+
+ if (!tty->stopped || tty->flow_stopped)
return;
- }
tty->stopped = 0;
+ spin_lock_irqsave(&tty->ctrl_lock, flags);
if (tty->link && tty->link->packet) {
tty->ctrl_status &= ~TIOCPKT_STOP;
tty->ctrl_status |= TIOCPKT_START;
@@ -991,6 +999,14 @@ void start_tty(struct tty_struct *tty)
tty_wakeup(tty);
}
+void start_tty(struct tty_struct *tty)
+{
+ unsigned long flags;
+
+ spin_lock_irqsave(&tty->flow_lock, flags);
+ __start_tty(tty);
+ spin_unlock_irqrestore(&tty->flow_lock, flags);
+}
EXPORT_SYMBOL(start_tty);
/* We limit tty time update visibility to every 8 seconds or so. */
@@ -3031,6 +3047,7 @@ void initialize_tty_struct(struct tty_struct *tty,
INIT_WORK(&tty->hangup_work, do_tty_hangup);
mutex_init(&tty->atomic_write_lock);
spin_lock_init(&tty->ctrl_lock);
+ spin_lock_init(&tty->flow_lock);
INIT_LIST_HEAD(&tty->tty_files);
INIT_WORK(&tty->SAK_work, do_SAK_work);
diff --git a/include/linux/tty.h b/include/linux/tty.h
index a707196..af26171 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -252,6 +252,7 @@ struct tty_struct {
struct rw_semaphore termios_rwsem;
struct mutex winsize_mutex;
spinlock_t ctrl_lock;
+ spinlock_t flow_lock;
/* Termios values are protected by the termios rwsem */
struct ktermios termios, termios_locked;
struct termiox *termiox; /* May be NULL for unsupported */
@@ -261,7 +262,7 @@ struct tty_struct {
unsigned long flags;
int count;
struct winsize winsize; /* winsize_mutex */
- int stopped;
+ int stopped; /* flow_lock */
int flow_stopped;
int hw_stopped;
int packet;
@@ -400,7 +401,9 @@ extern int tty_paranoia_check(struct tty_struct *tty, struct inode *inode,
extern char *tty_name(struct tty_struct *tty, char *buf);
extern void tty_wait_until_sent(struct tty_struct *tty, long timeout);
extern int tty_check_change(struct tty_struct *tty);
+extern void __stop_tty(struct tty_struct *tty);
extern void stop_tty(struct tty_struct *tty);
+extern void __start_tty(struct tty_struct *tty);
extern void start_tty(struct tty_struct *tty);
extern int tty_register_driver(struct tty_driver *driver);
extern int tty_unregister_driver(struct tty_driver *driver);
diff --git a/include/linux/tty_driver.h b/include/linux/tty_driver.h
index e48c608..92e337c 100644
--- a/include/linux/tty_driver.h
+++ b/include/linux/tty_driver.h
@@ -152,6 +152,8 @@
* This routine notifies the tty driver that it should stop
* outputting characters to the tty device.
*
+ * Called with ->flow_lock held. Serialized with start() method.
+ *
* Optional:
*
* Note: Call stop_tty not this method.
@@ -161,6 +163,8 @@
* This routine notifies the tty driver that it resume sending
* characters to the tty device.
*
+ * Called with ->flow_lock held. Serialized with stop() method.
+ *
* Optional:
*
* Note: Call start_tty not this method.
--
2.1.0
--
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