svn commit: r294836 - head/sys/kern

[Konstantin Belousov]

Author: kib
Date: Tue Jan 26 14:46:39 2016
New Revision: 294836
URL: https://svnweb.freebsd.org/changeset/base/294836

Log:
  Don't clear the software flow control flag before draining for last
  close or assert the bug that it is clear when leaving.
  
  Remove an unrelated rotted comment that was attached to the buggy
  clearing.
  
  Since draining is not done in more cases, flushing is needed in more
  cases, so start fixing flushing:
  - do a full flush in ttydisc_close().  State what POSIX requires more
    clearly.  This was missing ttydevsw_pktnotify() calls to tell the
    devsw layer to flush.  Hardware tty drivers don't actually flush
    since they don't understand this API.
  - fix 2 missing wakeups in tty_flush().  Most of the wakeups here are
    unnecessary for last close.  But ttydisc_close() did one of the
    missing ones.
  
  This flow control bug ameliorated the design bug of requiring
  potentially unbounded waits in draining.  Software flow control is the
  easiest way to get an unbounded wait, and a long wait is sometimes
  actually useful.  Users can type the xoff character on the receiver
  and (if ixon is set on the sender) expect the output to be held until
  the user is ready for more.
  
  Hardware flow control can also give the unbounded wait, and this bug
  didn't affect hardware flow control.  Unbounded waits from hardware
  flow control take a more unusual configuration.  E.g., a terminal
  program that controls the modem status lines, or unplugging the cable
  in a configuration where this doesn't break the connection.
  
  The design bug is still ameliorated by a newer bug in draining for
  last close -- the 1 second timeout.  E.g., if the user types the
  xoff character and the sender reaches last close, then output is
  not resumed and the wait times out after just 1 second.  This is
  broken, but preferable to an unbounded wait.  Before this change,
  the output was resumed immediately and usually completed.
  
  Submitted by:	bde
  MFC after:	2 weeks

Modified:
  head/sys/kern/tty.c
  head/sys/kern/tty_ttydisc.c

Modified: head/sys/kern/tty.c
==============================================================================
--- head/sys/kern/tty.c	Tue Jan 26 14:45:25 2016	(r294835)
+++ head/sys/kern/tty.c	Tue Jan 26 14:46:39 2016	(r294836)
@@ -201,7 +201,6 @@ ttydev_leave(struct tty *tp)
 		constty_clear();
 
 	/* Drain any output. */
-	MPASS((tp->t_flags & TF_STOPPED) == 0);
 	if (!tty_gone(tp))
 		tty_drain(tp, 1);
 
@@ -352,11 +351,7 @@ ttydev_close(struct cdev *dev, int fflag
 	if (fflag & FREVOKE)
 		tty_flush(tp, FWRITE);
 
-	/*
-	 * This can only be called once. The callin and the callout
-	 * devices cannot be opened at the same time.
-	 */
-	tp->t_flags &= ~(TF_EXCLUDE|TF_STOPPED);
+	tp->t_flags &= ~TF_EXCLUDE;
 
 	/* Properly wake up threads that are stuck - revoke(). */
 	tp->t_revokecnt++;
@@ -1456,12 +1451,15 @@ tty_flush(struct tty *tp, int flags)
 		tp->t_flags &= ~TF_HIWAT_OUT;
 		ttyoutq_flush(&tp->t_outq);
 		tty_wakeup(tp, FWRITE);
-		if (!tty_gone(tp))
+		if (!tty_gone(tp)) {
+			ttydevsw_outwakeup(tp);
 			ttydevsw_pktnotify(tp, TIOCPKT_FLUSHWRITE);
+		}
 	}
 	if (flags & FREAD) {
 		tty_hiwat_in_unblock(tp);
 		ttyinq_flush(&tp->t_inq);
+		tty_wakeup(tp, FREAD);
 		if (!tty_gone(tp)) {
 			ttydevsw_inwakeup(tp);
 			ttydevsw_pktnotify(tp, TIOCPKT_FLUSHREAD);

Modified: head/sys/kern/tty_ttydisc.c
==============================================================================
--- head/sys/kern/tty_ttydisc.c	Tue Jan 26 14:45:25 2016	(r294835)
+++ head/sys/kern/tty_ttydisc.c	Tue Jan 26 14:46:39 2016	(r294836)
@@ -94,14 +94,11 @@ ttydisc_close(struct tty *tp)
 	/* Clean up our flags when leaving the discipline. */
 	tp->t_flags &= ~(TF_STOPPED|TF_HIWAT|TF_ZOMBIE);
 
-	/* POSIX states we should flush when close() is called. */
-	ttyinq_flush(&tp->t_inq);
-	ttyoutq_flush(&tp->t_outq);
-
-	if (!tty_gone(tp)) {
-		ttydevsw_inwakeup(tp);
-		ttydevsw_outwakeup(tp);
-	}
+	/*
+	 * POSIX states that we must drain output and flush input on
+	 * last close.  Draining has already been done if possible.
+	 */
+	tty_flush(tp, FREAD | FWRITE);
 
 	if (ttyhook_hashook(tp, close))
 		ttyhook_close(tp);