tty: Destroy ldisc instance on hangup
Currently, when the tty is hungup, the ldisc is re-instanced; ie., the
current instance is destroyed and a new instance is created. The purpose
of this design was to guarantee a valid, open ldisc for the lifetime of
the tty.
However, now that tty buffers are owned by and have lifetime equivalent
to the tty_port (since v3.10), any data received immediately after the
ldisc is re-instanced may cause continued driver i/o operations
concurrently with the driver's hangup() operation. For drivers that
shutdown h/w on hangup, this is unexpected and usually bad. For example,
the serial core may free the xmit buffer page concurrently with an
in-progress write() operation (triggered by echo).
With the existing stable and robust ldisc reference handling, the
cleaned-up tty_reopen(), the straggling unsafe ldisc use cleaned up, and
the preparation to properly handle a NULL tty->ldisc, the ldisc instance
can be destroyed and only re-instanced when the tty is re-opened.
If the tty was opened as /dev/console or /dev/tty0, the original behavior
of re-instancing the ldisc is retained (the 'reinit' parameter to
tty_ldisc_hangup() is true). This is required since those file descriptors
are never hungup.
This patch has neglible impact on userspace; the tty file_operations ptr
is changed to point to the hungup file operations _before_ the ldisc
instance is destroyed, so only racing file operations might now retrieve
a NULL ldisc reference (which is simply handled as if the hungup file
operation had been called instead -- see "tty: Prepare for destroying
line discipline on hangup").
This resolves a long-standing FIXME and several crash reports.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index e18c8e8..c6f970d 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -257,6 +257,9 @@
* reference to it. If the line discipline is in flux then
* wait patiently until it changes.
*
+ * Returns: NULL if the tty has been hungup and not re-opened with
+ * a new file descriptor, otherwise valid ldisc reference
+ *
* Note: Must not be called from an IRQ/timer context. The caller
* must also be careful not to hold other locks that will deadlock
* against a discipline change, such as an existing ldisc reference
@@ -642,14 +645,15 @@
* @disc: line discipline to reinitialize
*
* Completely reinitialize the line discipline state, by closing the
- * current instance and opening a new instance. If an error occurs opening
- * the new non-N_TTY instance, the instance is dropped and tty->ldisc reset
- * to NULL. The caller can then retry with N_TTY instead.
+ * current instance, if there is one, and opening a new instance. If
+ * an error occurs opening the new non-N_TTY instance, the instance
+ * is dropped and tty->ldisc reset to NULL. The caller can then retry
+ * with N_TTY instead.
*
* Returns 0 if successful, otherwise error code < 0
*/
-static int tty_ldisc_reinit(struct tty_struct *tty, int disc)
+int tty_ldisc_reinit(struct tty_struct *tty, int disc)
{
struct tty_ldisc *ld;
int retval;
@@ -693,11 +697,9 @@
* tty itself so we must be careful about locking rules.
*/
-void tty_ldisc_hangup(struct tty_struct *tty)
+void tty_ldisc_hangup(struct tty_struct *tty, bool reinit)
{
struct tty_ldisc *ld;
- int reset = tty->driver->flags & TTY_DRIVER_RESET_TERMIOS;
- int err = 0;
tty_ldisc_debug(tty, "%p: hangup\n", tty->ldisc);
@@ -725,25 +727,17 @@
*/
tty_ldisc_lock(tty, MAX_SCHEDULE_TIMEOUT);
- if (tty->ldisc) {
-
- /* At this point we have a halted ldisc; we want to close it and
- reopen a new ldisc. We could defer the reopen to the next
- open but it means auditing a lot of other paths so this is
- a FIXME */
- if (reset == 0)
- err = tty_ldisc_reinit(tty, tty->termios.c_line);
-
- /* If the re-open fails or we reset then go to N_TTY. The
- N_TTY open cannot fail */
- if (reset || err < 0)
- tty_ldisc_reinit(tty, N_TTY);
- }
- tty_ldisc_unlock(tty);
- if (reset)
+ if (tty->driver->flags & TTY_DRIVER_RESET_TERMIOS)
tty_reset_termios(tty);
- tty_ldisc_debug(tty, "%p: re-opened\n", tty->ldisc);
+ if (tty->ldisc) {
+ if (reinit) {
+ if (tty_ldisc_reinit(tty, tty->termios.c_line) < 0)
+ tty_ldisc_reinit(tty, N_TTY);
+ } else
+ tty_ldisc_kill(tty);
+ }
+ tty_ldisc_unlock(tty);
}
/**