tty: Track flip buffer memory limit atomically

Lockless flip buffers require atomically updating the bytes-in-use
watermark.

The pty driver also peeks at the watermark value to limit
memory consumption to a much lower value than the default; query
the watermark with new fn, tty_buffer_space_avail().

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 3 insertions, 7 deletions

diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index 1b39dd639ee9..b38a28bd9511 100644
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -89,17 +89,13 @@ static void pty_unthrottle(struct tty_struct *tty)
  *      pty_space       -       report space left for writing
  *      @to: tty we are writing into
  *
- *      The tty buffers allow 64K but we sneak a peak and clip at 8K this
- *      allows a lot of overspill room for echo and other fun messes to
- *      be handled properly
+ *      Limit the buffer space used by ptys to 8k.
  */
 
 static int pty_space(struct tty_struct *to)
 {
-        int n = 8192 - to->port->buf.memory_used;
-        if (n < 0)
-                return 0;
-        return n;
+        int n = tty_buffer_space_avail(to->port);
+        return min(n, 8192);
 }
 
 /**