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tty: fix ldisc lock inversion trace

PATCH In review Alan Cox 2 participants 2 messages May 24, 2012 – May 25, 2012 TTY layer and serial drivers

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Alan Cox

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#0 lore raw

PATCH 1/1 tty: fix ldisc lock inversion trace

From: Alan Cox <[email protected]>

This is caused by tty_release using tty_lock_pair to lock both sides of the pty/tty pair, and then tty_ldisc_release dropping and relocking one side only. We can drop both fine, so drop both to avoid any lock ordering concerns.

Signed-off-by Alan Cox <[email protected]>

---

 drivers/tty/tty_ldisc.c |    7 +++++--
 1 files changed, 5 insertions(+), 2 deletions(-)
drivers/tty/tty_ldisc.c +5 -2
index 173a900..833e851 100644
@@ -912,10 +912,13 @@ void tty_ldisc_release(struct tty_struct *tty, struct tty_struct *o_tty)
 	 * race with the set_ldisc code path.
 	 */
 
-	tty_unlock(tty);
+	/* We don't give a monkeys' about the o_tty lock here but we
+	   must do both to avoid lock inversions against another single
+	   locking case */
+	tty_unlock_pair(tty, o_tty);
 	tty_ldisc_halt(tty);
 	tty_ldisc_flush_works(tty);
-	tty_lock(tty);
+	tty_lock_pair(tty, o_tty);
 
 	mutex_lock(&tty->ldisc_mutex);
 	/*
#1 lore raw
Quoted 21 lines · Alan Cox

From: Alan Cox <[email protected]>

This is caused by tty_release using tty_lock_pair to lock both sides of the pty/tty pair, and then tty_ldisc_release dropping and relocking one side only. We can drop both fine, so drop both to avoid any lock ordering concerns.

Signed-off-by Alan Cox <[email protected]>

---

 drivers/tty/tty_ldisc.c |    7 +++++--

 1 files changed, 5 insertions(+), 2 deletions(-)
drivers/tty/tty_ldisc.c +0 -0
index 173a900..833e851 100644
@@ -912,10 +912,13 @@ void tty_ldisc_release(struct tty_struct *tty, struct tty_struct *o_tty)
         * race with the set_ldisc code path.
         */

drivers/tty/tty_ldisc.c void tty_ldisc_release(struct tty_struct *tty, struct tty_struct *o_tty) · line 921

Quoted 10 lines
drivers/tty/tty_ldisc.c +5 -2
 
-       tty_unlock(tty);
+       /* We don't give a monkeys' about the o_tty lock here but we
+          must do both to avoid lock inversions against another single
+          locking case */
+       tty_unlock_pair(tty, o_tty);
        tty_ldisc_halt(tty);
        tty_ldisc_flush_works(tty);
-       tty_lock(tty);
+       tty_lock_pair(tty, o_tty);

This still may cause lockdep warning by the following line:

             if (o_tty)
                    tty_ldisc_release(o_tty, NULL);

The tty_unlock_pair in 'tty_ldisc_release(o_tty, NULL)' may not match the previous tty_lock_pair. If tty > o_tty, then lockdep warning will be triggered since the lock to be released is not at the top of current lock stack.

Thanks,

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