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

2 files changed, 10 insertions, 0 deletions

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'},
 };