[PATCH 21/23] hvc_console: Fix race between hvc_close and hvc_remove

Anton Blanchard anton at samba.org
Tue Feb 8 08:16:00 EST 2011


> From: Amit Shah <amit.shah at redhat.com>
> commit e74d098c66543d0731de62eb747ccd5b636a6f4c upstream.
> Alan pointed out a race in the code where hvc_remove is invoked. The
> recent virtio_console work is the first user of hvc_remove().

I faintly remember this bug caused boot issues and the following patch
must be applied to fix it.


commit 320718ee074acce5ffced6506cb51af1388942aa
Author: Anton Blanchard <anton at samba.org>
Date:   Tue Apr 6 21:42:38 2010 +1000

    hvc_console: Fix race between hvc_close and hvc_remove
    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);
                    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:
            if (--hp->count == 0) {
                    /* Put the ref obtained in hvc_open() */
    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 at samba.org>
    Acked-by: Amit Shah <amit.shah at redhat.com>
    Signed-off-by: Rusty Russell <rusty at rustcorp.com.au>

diff --git a/drivers/char/hvc_console.c b/drivers/char/hvc_console.c
index d3890e8..35cca4c 100644
--- a/drivers/char/hvc_console.c
+++ b/drivers/char/hvc_console.c
@@ -368,16 +368,12 @@ 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);

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