From 324c11dfce9afab0ca49b24f299f03923ae8167b Mon Sep 17 00:00:00 2001
From: Taylor R Campbell <riastradh@NetBSD.org>
Date: Mon, 31 Jan 2022 00:54:47 +0000
Subject: [PATCH] tty(9): New ttycancel function.

This causes any current and future ttyopens to fail until ttyclose.

This is necessary for revoke to work reliably for device detach like
ucom(4) removable USB devices.  A tty driver for a removable device
needs some way to interrupt a pending .d_open so it returns promptly.
But ttyclose only interrupts ttyopen if it's already sleeping; it
won't cause a concurrent .d_open call which _will call_ but _hasn't
yet called_ ttyopen to avoid sleeping.  Using ttycancel in the tty
driver's .d_cancel makes this work.
---
 sys/kern/tty.c | 19 ++++++++++++++++++-
 sys/sys/tty.h  |  3 +++
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/sys/kern/tty.c b/sys/kern/tty.c
index 32a4c4d47b65..6ed86dbb2b3b 100644
--- a/sys/kern/tty.c
+++ b/sys/kern/tty.c
@@ -419,6 +419,21 @@ ttylopen(dev_t device, struct tty *tp)
 	return (0);
 }
 
+/*
+ * Interrupt any pending I/O and make it fail.  Used before close to
+ * interrupt pending open/read/write/&c. and make it fail promptly.
+ */
+void
+ttycancel(struct tty *tp)
+{
+
+	mutex_spin_enter(&tty_lock);
+	tp->t_state |= TS_CANCEL;
+	cv_broadcast(&tp->t_outcv);
+	cv_broadcast(&tp->t_rawcv);
+	mutex_spin_exit(&tty_lock);
+}
+
 /*
  * Handle close() on a tty line: flush and set to initial state,
  * bumping generation number so that pending read/write calls
@@ -2750,7 +2765,9 @@ ttysleep(struct tty *tp, kcondvar_t *cv, bool catch_p, int timo)
 	KASSERT(mutex_owned(&tty_lock));
 
 	gen = tp->t_gen;
-	if (cv == NULL)
+	if (ISSET(tp->t_state, TS_CANCEL))
+		error = ERESTART;
+	else if (cv == NULL)
 		error = kpause("ttypause", catch_p, timo, &tty_lock);
 	else if (catch_p)
 		error = cv_timedwait_sig(cv, &tty_lock, timo);
diff --git a/sys/sys/tty.h b/sys/sys/tty.h
index 47a8a29e6c4f..3b4c403160e5 100644
--- a/sys/sys/tty.h
+++ b/sys/sys/tty.h
@@ -207,6 +207,8 @@ struct tty {
 #define	TS_KERN_ONLY	0x10000		/* Device is accessible by kernel
 					 * only, deny all userland access */
 
+#define	TS_CANCEL	0x20000		/* I/O cancelled pending close. */
+
 /* Character type information. */
 #define	ORDINARY	0
 #define	CONTROL		1
@@ -281,6 +283,7 @@ void	 ttwakeup(struct tty *);
 int	 ttwrite(struct tty *, struct uio *, int);
 void	 ttychars(struct tty *);
 int	 ttycheckoutq(struct tty *, int);
+void	 ttycancel(struct tty *);
 int	 ttyclose(struct tty *);
 void	 ttyflush(struct tty *, int);
 void	 ttygetinfo(struct tty *, int, char *, size_t);