tty: Simplify flip buffer list with 0-sized sentinel

Use a 0-sized sentinel to avoid assigning the head ptr from
the driver side thread. This also eliminates testing head/tail
for NULL.

When the sentinel is first 'consumed' by the buffer work
(or by tty_buffer_flush()), it is detached from the list but not
freed nor added to the free list. Both buffer work and
tty_buffer_flush() continue to preserve at least 1 flip buffer
to which head & tail is pointed.

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

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/tty.h b/include/linux/tty.h
index 5043b12f23ea..2e93eb831c61 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -67,6 +67,7 @@ static inline char *flag_buf_ptr(struct tty_buffer *b, int ofs)
 struct tty_bufhead {
         struct work_struct work;
         spinlock_t lock;
+        struct tty_buffer sentinel;
         struct tty_buffer *head;        /* Queue head */
         struct tty_buffer *tail;        /* Active buffer */
         struct llist_head free;         /* Free queue head */