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-rw-r--r--drivers/char/pty.c53
1 files changed, 40 insertions, 13 deletions
diff --git a/drivers/char/pty.c b/drivers/char/pty.c
index 5acd29e6e043..daebe1ba43d4 100644
--- a/drivers/char/pty.c
+++ b/drivers/char/pty.c
@@ -95,23 +95,34 @@ static void pty_unthrottle(struct tty_struct *tty)
* a count.
*
* FIXME: Our pty_write method is called with our ldisc lock held but
- * not our partners. We can't just take the other one blindly without
- * risking deadlocks.
+ * not our partners. We can't just wait on the other one blindly without
+ * risking deadlocks. At some point when everything has settled down we need
+ * to look into making pty_write at least able to sleep over an ldisc change.
+ *
+ * The return on no ldisc is a bit counter intuitive but the logic works
+ * like this. During an ldisc change the other end will flush its buffers. We
+ * thus return the full length which is identical to the case where we had
+ * proper locking and happened to queue the bytes just before the flush during
+ * the ldisc change.
*/
static int pty_write(struct tty_struct *tty, const unsigned char *buf,
int count)
{
struct tty_struct *to = tty->link;
- int c;
+ struct tty_ldisc *ld;
+ int c = count;
if (!to || tty->stopped)
return 0;
-
- c = to->receive_room;
- if (c > count)
- c = count;
- to->ldisc->ops->receive_buf(to, buf, NULL, c);
-
+ ld = tty_ldisc_ref(to);
+
+ if (ld) {
+ c = to->receive_room;
+ if (c > count)
+ c = count;
+ ld->ops->receive_buf(to, buf, NULL, c);
+ tty_ldisc_deref(ld);
+ }
return c;
}
@@ -145,14 +156,23 @@ static int pty_write_room(struct tty_struct *tty)
static int pty_chars_in_buffer(struct tty_struct *tty)
{
struct tty_struct *to = tty->link;
- int count;
+ struct tty_ldisc *ld;
+ int count = 0;
/* We should get the line discipline lock for "tty->link" */
- if (!to || !to->ldisc->ops->chars_in_buffer)
+ if (!to)
+ return 0;
+ /* We cannot take a sleeping reference here without deadlocking with
+ an ldisc change - but it doesn't really matter */
+ ld = tty_ldisc_ref(to);
+ if (ld == NULL)
return 0;
/* The ldisc must report 0 if no characters available to be read */
- count = to->ldisc->ops->chars_in_buffer(to);
+ if (ld->ops->chars_in_buffer)
+ count = ld->ops->chars_in_buffer(to);
+
+ tty_ldisc_deref(ld);
if (tty->driver->subtype == PTY_TYPE_SLAVE)
return count;
@@ -182,12 +202,19 @@ static void pty_flush_buffer(struct tty_struct *tty)
{
struct tty_struct *to = tty->link;
unsigned long flags;
+ struct tty_ldisc *ld;
if (!to)
return;
+ ld = tty_ldisc_ref(to);
+
+ /* The other end is changing discipline */
+ if (!ld)
+ return;
- if (to->ldisc->ops->flush_buffer)
+ if (ld->ops->flush_buffer)
to->ldisc->ops->flush_buffer(to);
+ tty_ldisc_deref(ld);
if (to->packet) {
spin_lock_irqsave(&tty->ctrl_lock, flags);