linux-kernel - [PATCH 6/6] hvc_console: Fix race between hvc_close and hvc

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Message-Id: <1276554087-3632-7-git-send-email-jkacur@redhat.com>
Date:	Tue, 15 Jun 2010 00:21:27 +0200
From:	John Kacur <jkacur@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	rt-users <linux-rt-users@...r.kernel.org>,
	Clark Williams <williams@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>
Subject: [PATCH 6/6] hvc_console: Fix race between hvc_close and hvc_remove

From: Anton Blanchard <anton@...ba.org>

I don't claim to understand the tty layer, but it seems like hvc_open and
hvc_close should be balanced in their kref reference counting.

Right now we get a kref every call to hvc_open:

        if (hp->count++ > 0) {
                tty_kref_get(tty); <----- here
                spin_unlock_irqrestore(&hp->lock, flags);
                hvc_kick();
                return 0;
        } /* else count == 0 */

        tty->driver_data = hp;

        hp->tty = tty_kref_get(tty); <------ or here if hp->count was 0

But hvc_close has:

        tty_kref_get(tty);

        if (--hp->count == 0) {
...
                /* Put the ref obtained in hvc_open() */
                tty_kref_put(tty);
...
        }

        tty_kref_put(tty);

Since the outside kref get/put balance we only do a single kref_put when
count reaches 0.

The patch below changes things to call tty_kref_put once for every
hvc_close call, and with that my machine boots fine.

Signed-off-by: Anton Blanchard <anton@...ba.org>
Acked-by: Amit Shah <amit.shah@...hat.com>
Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
Upstream-commit: 320718ee074acce5ffced6506cb51af1388942aa
Signed-off-by: John Kacur <jkacur@...hat.com>
---
 drivers/char/hvc_console.c |    4 ----
 1 files changed, 0 insertions(+), 4 deletions(-)

diff --git a/drivers/char/hvc_console.c b/drivers/char/hvc_console.c
index da70f68..2898084 100644
--- a/drivers/char/hvc_console.c
+++ b/drivers/char/hvc_console.c
@@ -367,16 +367,12 @@ static void hvc_close(struct tty_struct *tty, struct file * filp)
 	hp = tty->driver_data;
 
 	spin_lock_irqsave(&hp->lock, flags);
-	tty_kref_get(tty);
 
 	if (--hp->count == 0) {
 		/* We are done with the tty pointer now. */
 		hp->tty = NULL;
 		spin_unlock_irqrestore(&hp->lock, flags);
 
-		/* Put the ref obtained in hvc_open() */
-		tty_kref_put(tty);
-
 		if (hp->ops->notifier_del)
 			hp->ops->notifier_del(hp, hp->data);
 
-- 
1.6.6.1

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