linux-kernel - [PATCH 3/4] tty: Lock tty pair in tty_init

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Message-Id: <20180829022353.23568-4-dima@arista.com>
Date:   Wed, 29 Aug 2018 03:23:52 +0100
From:   Dmitry Safonov <dima@...sta.com>
To:     linux-kernel@...r.kernel.org
Cc:     Dmitry Safonov <0x7f454c46@...il.com>,
        Dmitry Safonov <dima@...sta.com>,
        Daniel Axtens <dja@...ens.net>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Michael Neuling <mikey@...ling.org>,
        Mikulas Patocka <mpatocka@...hat.com>,
        Pasi Kärkkäinen <pasik@....fi>,
        Peter Hurley <peter@...leysoftware.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Tan Xiaojun <tanxiaojun@...wei.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>
Subject: [PATCH 3/4] tty: Lock tty pair in tty_init_dev()

It's safe to not lock both here - done to silence attempt lockdep assert in
tty_ldisc_open(), which will be added with following patch.

Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Jiri Slaby <jslaby@...e.com>
Signed-off-by: Dmitry Safonov <dima@...sta.com>
---
 drivers/tty/tty_io.c    | 5 ++---
 drivers/tty/tty_ldisc.c | 6 ++----
 include/linux/tty.h     | 4 ++++
 3 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 3ef8b977b167..a94005f915b3 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1342,7 +1342,7 @@ struct tty_struct *tty_init_dev(struct tty_driver *driver, int idx)
 			"%s: %s driver does not set tty->port. This will crash the kernel later. Fix the driver!\n",
 			__func__, tty->driver->name);
 
-	retval = tty_ldisc_lock(tty, 5 * HZ);
+	retval = tty_ldisc_lock_pair_timeout(tty, tty->link, 5 * HZ);
 	if (retval)
 		goto err_release_lock;
 	tty->port->itty = tty;
@@ -1353,9 +1353,9 @@ struct tty_struct *tty_init_dev(struct tty_driver *driver, int idx)
 	 * to decrement the use counts, as release_tty doesn't care.
 	 */
 	retval = tty_ldisc_setup(tty, tty->link);
+	tty_ldisc_unlock_pair(tty, tty->link);
 	if (retval)
 		goto err_release_tty;
-	tty_ldisc_unlock(tty);
 	/* Return the tty locked so that it cannot vanish under the caller */
 	return tty;
 
@@ -1368,7 +1368,6 @@ struct tty_struct *tty_init_dev(struct tty_driver *driver, int idx)
 
 	/* call the tty release_tty routine to clean out this slot */
 err_release_tty:
-	tty_ldisc_unlock(tty);
 	tty_info_ratelimited(tty, "ldisc open failed (%d), clearing slot %d\n",
 			     retval, idx);
 err_release_lock:
diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index fc4c97cae01e..b72266461c00 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -340,8 +340,7 @@ void tty_ldisc_unlock(struct tty_struct *tty)
 	__tty_ldisc_unlock(tty);
 }
 
-static int
-tty_ldisc_lock_pair_timeout(struct tty_struct *tty, struct tty_struct *tty2,
+int tty_ldisc_lock_pair_timeout(struct tty_struct *tty, struct tty_struct *tty2,
 			    unsigned long timeout)
 {
 	int ret;
@@ -381,8 +380,7 @@ static void tty_ldisc_lock_pair(struct tty_struct *tty, struct tty_struct *tty2)
 	tty_ldisc_lock_pair_timeout(tty, tty2, MAX_SCHEDULE_TIMEOUT);
 }
 
-static void tty_ldisc_unlock_pair(struct tty_struct *tty,
-				  struct tty_struct *tty2)
+void tty_ldisc_unlock_pair(struct tty_struct *tty, struct tty_struct *tty2)
 {
 	__tty_ldisc_unlock(tty);
 	if (tty2)
diff --git a/include/linux/tty.h b/include/linux/tty.h
index c56e3978b00f..5efb8f87ffdc 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -409,6 +409,10 @@ extern void tty_kclose(struct tty_struct *tty);
 extern int tty_dev_name_to_number(const char *name, dev_t *number);
 extern int tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout);
 extern void tty_ldisc_unlock(struct tty_struct *tty);
+extern int tty_ldisc_lock_pair_timeout(struct tty_struct *tty,
+		struct tty_struct *tty2, unsigned long timeout);
+extern void tty_ldisc_unlock_pair(struct tty_struct *tty,
+		struct tty_struct *tty2);
 #else
 static inline void tty_kref_put(struct tty_struct *tty)
 { }
-- 
2.13.6