fix serial buffer memory leak

Patch c5c34d4862 (tty: flush flip buffer on
ldisc input queue flush) introduces a race condition which can lead to memory
leaks.

The problem can be triggered when tcflush() is called when data are being
pushed to the line discipline driver by flush_to_ldisc().

flush_to_ldisc() releases tty->buf.lock when calling the line discipline
receive_buf function. At that poing tty_buffer_flush() kicks in and sets both
tty->buf.head and tty->buf.tail to NULL. When flush_to_ldisc() finishes, it
restores tty->buf.head but doesn't touch tty->buf.tail. This corrups the
buffer queue, and the next call to tty_buffer_request_room() will allocate a
new buffer and overwrite tty->buf.head. The previous buffer is then lost
forever without being released.

(Thanks to Laurent for the above text, for finding, disgnosing and reporting
the bug)

- Use tty->flags bits for the flush status.

- Wait for the flag to clear again before returning

- Fix the doc error noted

- Fix flush of empty queue leaving stale flushpending

[akpm@linux-foundation.org: cleanup]
Signed-off-by: Alan Cox <alan@redhat.com>
Acked-by: Paul Fulghum <paulkf@microgate.com>
Cc: Laurent Pinchart <laurentp@cse-semaphore.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/tty.h

@@ -274,6 +274,8 @@ struct tty_struct {
 #define TTY_PTY_LOCK 		16	/* pty private */
 #define TTY_NO_WRITE_SPLIT 	17	/* Preserve write boundaries to driver */
 #define TTY_HUPPED 		18	/* Post driver->hangup() */
+#define TTY_FLUSHING		19	/* Flushing to ldisc in progress */
+#define TTY_FLUSHPENDING	20	/* Queued buffer flush pending */
 
 #define TTY_WRITE_FLUSH(tty) tty_write_flush((tty))