tty: Use non-atomic state to signal flip buffer flush pending

Atomic bit ops are no longer required to indicate a flip buffer
flush is pending, as the flush_mutex is sufficient barrier.

Remove the unnecessary port .iflags field and localize flip buffer
state to struct tty_bufhead.

Signed-off-by: Peter Hurley <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/tty_buffer.c

···
 	struct tty_port *port = tty->port;
 	struct tty_bufhead *buf = &port->buf;
 
-	set_bit(TTYP_FLUSHPENDING, &port->iflags);
+	buf->flushpending = 1;
 
 	mutex_lock(&buf->flush_mutex);
 	__tty_buffer_flush(port);
-	clear_bit(TTYP_FLUSHPENDING, &port->iflags);
+	buf->flushpending = 0;
 	mutex_unlock(&buf->flush_mutex);
 }
 
···
 		int count;
 
 		/* Ldisc or user is trying to flush the buffers. */
-		if (test_bit(TTYP_FLUSHPENDING, &port->iflags))
+		if (buf->flushpending)
 			break;
 
 		count = head->commit - head->read;
···
 	buf->tail = &buf->sentinel;
 	init_llist_head(&buf->free);
 	atomic_set(&buf->memory_used, 0);
+	buf->flushpending = 0;
 	INIT_WORK(&buf->work, flush_to_ldisc);
 }
 

include/linux/tty.h

···
 	struct tty_buffer *head;	/* Queue head */
 	struct work_struct work;
 	struct mutex flush_mutex;
+	unsigned int flushpending:1;
 	struct tty_buffer sentinel;
 	struct llist_head free;		/* Free queue head */
 	atomic_t	   memory_used; /* In-use buffers excluding free list */
···
 	wait_queue_head_t	close_wait;	/* Close waiters */
 	wait_queue_head_t	delta_msr_wait;	/* Modem status change */
 	unsigned long		flags;		/* TTY flags ASY_*/
-	unsigned long		iflags;		/* TTYP_ internal flags */
-#define TTYP_FLUSHPENDING		2  /* Queued buffer flush pending */
 	unsigned char		console:1,	/* port is a console */
 				low_latency:1;	/* direct buffer flush */
 	struct mutex		mutex;		/* Locking */