tty: Use lockless flip buffer free list

In preparation for lockless flip buffers, make the flip buffer
free list lockless.

NB: using llist is not the optimal solution, as the driver and
buffer work may contend over the llist head unnecessarily. However,
test measurements indicate this contention is low.

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

1 files changed, 6 insertions, 2 deletions

diff --git a/include/linux/tty.h b/include/linux/tty.h
index 87bbaa31ebf5..5043b12f23ea 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -11,6 +11,7 @@
 #include <linux/tty_flags.h>
 #include <uapi/linux/tty.h>
 #include <linux/rwsem.h>
+#include <linux/llist.h>
 
 
 
@@ -30,7 +31,10 @@
 #define __DISABLED_CHAR '\0'
 
 struct tty_buffer {
-	struct tty_buffer *next;
+	union {
+		struct tty_buffer *next;
+		struct llist_node free;
+	};
 	int used;
 	int size;
 	int commit;
@@ -65,7 +69,7 @@ struct tty_bufhead {
 	spinlock_t lock;
 	struct tty_buffer *head;	/* Queue head */
 	struct tty_buffer *tail;	/* Active buffer */
-	struct tty_buffer *free;	/* Free queue head */
+	struct llist_head free;		/* Free queue head */
 	int memory_used;		/* Buffer space used excluding
 								free queue */
 };