Serialize write() calls on TTYs.

Just like the old TTY layer, the current MPSAFE TTY layer does not make
any attempt to serialize calls of write(). Data is copied into the
kernel in 256 (TTY_STACKBUF) byte chunks. If a write() call occurs at
the same time, the data may interleave. This is especially likely when
the TTY starts blocking, because the output queue reaches the high
watermark.

I've implemented this by adding a new flag, TTY_BUSY_OUT, which is used
to mark a TTY as having a thread stuck in write(). Because I don't want
non-blocking processes to be possibly blocked by a sleeping thread, I'm
still allowing it to bypass the protection. According to this message,
the Linux kernel returns EAGAIN in such cases, but I think that's a
little too restrictive:

	http://kerneltrap.org/index.php?q=mailarchive/linux-kernel/2007/5/2/85418/thread

PR:		kern/118287

4 files changed, 37 insertions, 6 deletions

diff --git a/sys/kern/tty.c b/sys/kern/tty.c
index d73683c..ee95958 100644
--- a/sys/kern/tty.c
+++ b/sys/kern/tty.c
@@ -438,15 +438,28 @@ ttydev_write(struct cdev *dev, struct uio *uio, int ioflag)
 
 	if (tp->t_termios.c_lflag & TOSTOP) {
 		error = tty_wait_background(tp, curthread, SIGTTOU);
-		if (error) {
-			tty_unlock(tp);
-			return (error);
-		}
+		if (error)
+			goto done;
 	}
 
-	error = ttydisc_write(tp, uio, ioflag);
-	tty_unlock(tp);
+	if (ioflag & IO_NDELAY && tp->t_flags & TF_BUSY_OUT) {
+		/* Allow non-blocking writes to bypass serialization. */
+		error = ttydisc_write(tp, uio, ioflag);
+	} else {
+		/* Serialize write() calls. */
+		while (tp->t_flags & TF_BUSY_OUT) {
+			error = tty_wait(tp, &tp->t_bgwait);
+			if (error)
+				goto done;
+		}
+ 
+ 		tp->t_flags |= TF_BUSY_OUT;
+		error = ttydisc_write(tp, uio, ioflag);
+ 		tp->t_flags &= ~TF_BUSY_OUT;
+		cv_broadcast(&tp->t_bgwait);
+	}
 
+done:	tty_unlock(tp);
 	return (error);
 }
 
@@ -1880,6 +1893,11 @@ static struct {
 	{ TF_ZOMBIE,		'Z' },
 	{ TF_HOOK,		's' },
 
+	/* Keep these together -> 'bi' and 'bo'. */
+	{ TF_BUSY,		'b' },
+	{ TF_BUSY_IN,		'i' },
+	{ TF_BUSY_OUT,		'o' },
+
 	{ 0,			'\0'},
 };
 
diff --git a/sys/sys/tty.h b/sys/sys/tty.h
index 218fd21..8935093 100644
--- a/sys/sys/tty.h
+++ b/sys/sys/tty.h
@@ -83,6 +83,9 @@ struct tty {
 #define	TF_BYPASS	0x04000	/* Optimized input path. */
 #define	TF_ZOMBIE	0x08000	/* Modem disconnect received. */
 #define	TF_HOOK		0x10000	/* TTY has hook attached. */
+#define	TF_BUSY_IN	0x20000	/* Process busy in read() -- not supported. */
+#define	TF_BUSY_OUT	0x40000	/* Process busy in write(). */
+#define	TF_BUSY		(TF_BUSY_IN|TF_BUSY_OUT)
 	unsigned int	t_revokecnt;	/* (t) revoke() count. */
 
 	/* Buffering mechanisms. */
diff --git a/usr.sbin/pstat/pstat.8 b/usr.sbin/pstat/pstat.8
index b92abd8..5e3760c 100644
--- a/usr.sbin/pstat/pstat.8
+++ b/usr.sbin/pstat/pstat.8
@@ -206,6 +206,11 @@ block mode input routine in use
 connection lost
 .It s
 i/o being snooped
+.It b
+busy in
+.Xr read 2
+or
+.Xr write 2
 .El
 .Pp
 The
diff --git a/usr.sbin/pstat/pstat.c b/usr.sbin/pstat/pstat.c
index 2cb52fe..8272eee 100644
--- a/usr.sbin/pstat/pstat.c
+++ b/usr.sbin/pstat/pstat.c
@@ -315,6 +315,11 @@ static struct {
 	{ TF_ZOMBIE,		'Z' },
 	{ TF_HOOK,		's' },
 
+	/* Keep these together -> 'bi' and 'bo'. */
+	{ TF_BUSY,		'b' },
+	{ TF_BUSY_IN,		'i' },
+	{ TF_BUSY_OUT,		'o' },
+
 	{ 0,			'\0'},
 };