tty: Make flush_to_ldisc() locking more robust

The locking logic in this function is extremely subtle, and it broke
when we started doing potentially concurrent 'flush_to_ldisc()' calls in
commit e043e42bdb66885b3ac10d27a01ccb9972e2b0a3 ("pty: avoid forcing
'low_latency' tty flag").

The code in flush_to_ldisc() used to set 'tty->buf.head' to NULL, with
the intention that this would then cause any other concurrent calls to
not do anything (locking note: we have to drop the buf.lock over the
call to ->receive_buf that can block, which is why we can have
concurrency here at all in the first place).

It also used to set the TTY_FLUSHING bit, which would then cause any
concurrent 'tty_buffer_flush()' to not free all the tty buffers and
clear 'tty->buf.tail'.  And with 'buf.head' being NULL, and 'buf.tail'
being non-NULL, new data would never touch 'buf.head'.

Does that sound a bit too subtle? It was.  If another concurrent call to
'flush_to_ldisc()' were to come in, the NULL buf.head would indeed cause
it to not process the buffer list, but it would still clear TTY_FLUSHING
afterwards, making the buffer protection against 'tty_buffer_flush()' no
longer work.

So this clears it all up.  We depend purely on TTY_FLUSHING for handling
re-entrancy, and stop playing games with the buffer list entirely.  In
fact, the buffer list handling is now robust enough that we could
probably stop doing the whole "protect against 'tty_buffer_flush()'"
thing entirely.

However, Alan also points out that we would probably be better off
simplifying the locking even further, and just take the tty ldisc_mutex
around all the buffer flushing calls.  That seems like a good idea, but
in the meantime this is a conceptually minimal fix (with the patch
itself being bigger than required just to clean the code up and make it
readable).

This fixes keyboard trouble under X:

	http://bugzilla.kernel.org/show_bug.cgi?id=14388

Reported-and-tested-by: Frédéric Meunier <fredlwm@gmail.com>
Reported-and-tested-by: Boyan <btanastasov@yahoo.co.uk>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Paul Fulghum <paulkf@microgate.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/char/tty_buffer.c

@@ -402,28 +402,26 @@ static void flush_to_ldisc(struct work_struct *work)
 		container_of(work, struct tty_struct, buf.work.work);
 	unsigned long 	flags;
 	struct tty_ldisc *disc;
-	struct tty_buffer *tbuf, *head;
-	char *char_buf;
-	unsigned char *flag_buf;
 
 	disc = tty_ldisc_ref(tty);
 	if (disc == NULL)	/*  !TTY_LDISC */
 		return;
 
 	spin_lock_irqsave(&tty->buf.lock, flags);
-	/* So we know a flush is running */
-	set_bit(TTY_FLUSHING, &tty->flags);
-	head = tty->buf.head;
-	if (head != NULL) {
-		tty->buf.head = NULL;
-		for (;;) {
-			int count = head->commit - head->read;
+
+	if (!test_and_set_bit(TTY_FLUSHING, &tty->flags)) {
+		struct tty_buffer *head;
+		while ((head = tty->buf.head) != NULL) {
+			int count;
+			char *char_buf;
+			unsigned char *flag_buf;
+
+			count = head->commit - head->read;
 			if (!count) {
 				if (head->next == NULL)
 					break;
-				tbuf = head;
-				head = head->next;
-				tty_buffer_free(tty, tbuf);
+				tty->buf.head = head->next;
+				tty_buffer_free(tty, head);
 				continue;
 			}
 			/* Ldisc or user is trying to flush the buffers
@@ -445,9 +443,9 @@ static void flush_to_ldisc(struct work_struct *work)
 							flag_buf, count);
 			spin_lock_irqsave(&tty->buf.lock, flags);
 		}
-		/* Restore the queue head */
-		tty->buf.head = head;
+		clear_bit(TTY_FLUSHING, &tty->flags);
 	}
+
 	/* We may have a deferred request to flush the input buffer,
 	   if so pull the chain under the lock and empty the queue */
 	if (test_bit(TTY_FLUSHPENDING, &tty->flags)) {
@@ -455,7 +453,6 @@ static void flush_to_ldisc(struct work_struct *work)
 		clear_bit(TTY_FLUSHPENDING, &tty->flags);
 		wake_up(&tty->read_wait);
 	}
-	clear_bit(TTY_FLUSHING, &tty->flags);
 	spin_unlock_irqrestore(&tty->buf.lock, flags);
 
 	tty_ldisc_deref(disc);