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>

2 files changed, 2 insertions, 2 deletions

diff --git a/include/linux/tty.h b/include/linux/tty.h
index 2e93eb831c61..7c124541f011 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -71,8 +71,7 @@ struct tty_bufhead {
         struct tty_buffer *head;        /* Queue head */
         struct tty_buffer *tail;        /* Active buffer */
         struct llist_head free;         /* Free queue head */
-        int memory_used;                /* Buffer space used excluding
-                                                                free queue */
+        atomic_t           memory_used; /* In-use buffers excluding free list */
 };
 /*
  * When a break, frame error, or parity error happens, these codes are
diff --git a/include/linux/tty_flip.h b/include/linux/tty_flip.h
index ad0303994c32..6944ed2ce692 100644
--- a/include/linux/tty_flip.h
+++ b/include/linux/tty_flip.h
@@ -1,6 +1,7 @@
 #ifndef _LINUX_TTY_FLIP_H
 #define _LINUX_TTY_FLIP_H
 
+extern int tty_buffer_space_avail(struct tty_port *port);
 extern int tty_buffer_request_room(struct tty_port *port, size_t size);
 extern int tty_insert_flip_string_flags(struct tty_port *port,
                 const unsigned char *chars, const char *flags, size_t size);