Fix tty hiwat handling a bit

- Introduce new defines TTHIWATMINSPACE, TTMINHIWAT for some magic values
  that are used in tty.c.
- Remove hiwat adjustments in ttwrite(). This fixes this codepath not
  being interrupt safe.
- Change ttysetwater() to keep at least TTHIWATMINSPACE space above the high
  water mark. This makes it consistent with ttycheckoutq(). Without this
  change, the hiwat adjustment change above causes deadlocks in pty.

ok kspillner@
commit it now deraadt@

1 files changed, 5 insertions, 14 deletions

diff --git a/sys/kern/tty.c b/sys/kern/tty.c
index c0ec4f0e259..354d14d52d2 100644
--- a/sys/kern/tty.c
+++ b/sys/kern/tty.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: tty.c,v 1.121 2015/02/10 21:56:10 miod Exp $	*/
+/*	$OpenBSD: tty.c,v 1.122 2015/07/20 22:28:57 sf Exp $	*/
 /*	$NetBSD: tty.c,v 1.68.4.2 1996/06/06 16:04:52 thorpej Exp $	*/
 
 /*-
@@ -1688,7 +1688,7 @@ ttycheckoutq(struct tty *tp, int wait)
 	hiwat = tp->t_hiwat;
 	s = spltty();
 	oldsig = wait ? curproc->p_siglist : 0;
-	if (tp->t_outq.c_cc > hiwat + 200)
+	if (tp->t_outq.c_cc > hiwat + TTHIWATMINSPACE)
 		while (tp->t_outq.c_cc > hiwat) {
 			ttstart(tp);
 			if (wait == 0 || curproc->p_siglist != oldsig) {
@@ -1823,7 +1823,7 @@ loop:
 					tp->t_rocount = 0;
 					if (ttyoutput(*cp, tp) >= 0) {
 						/* out of space */
-						goto overfull;
+						goto ovhiwat;
 					}
 					cp++;
 					cc--;
@@ -1849,7 +1849,7 @@ loop:
 			tp->t_outcc += ce;
 			if (i > 0) {
 				/* out of space */
-				goto overfull;
+				goto ovhiwat;
 			}
 			if (ISSET(tp->t_lflag, FLUSHO) ||
 			    tp->t_outq.c_cc > hiwat)
@@ -1869,15 +1869,6 @@ done:
 		explicit_bzero(obuf, obufcc);
 	return (error);
 
-overfull:
-	/*
-	 * Since we are using ring buffers, if we can't insert any more into
-	 * the output queue, we can assume the ring is full and that someone
-	 * forgot to set the high water mark correctly.  We set it and then
-	 * proceed as normal.
-	 */
-	hiwat = tp->t_outq.c_cc - 1;
-
 ovhiwat:
 	ttstart(tp);
 	s = spltty();
@@ -2114,7 +2105,7 @@ ttsetwater(struct tty *tp)
 	cps = tp->t_ospeed / 10;
 	tp->t_lowat = x = CLAMP(cps / 2, TTMAXLOWAT, TTMINLOWAT);
 	x += cps;
-	tp->t_hiwat = CLAMP(x, tp->t_outq.c_cn, 100);
+	tp->t_hiwat = CLAMP(x, tp->t_outq.c_cn - TTHIWATMINSPACE, TTMINHIWAT);
 #undef	CLAMP
 }