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    SubjectRe: [PATCH 4/7] hvc_console: Fix race between hvc_close and hvc_remove

    Hi,

    > > Looking at the commit e74d098c66543d0731de62eb747ccd5b636a6f4c,
    > > i see that for every tty_kref_get() there is a corresponding
    > > tty_kref_put() except maybe for the one in the following patch snippet
    > >
    > > spin_lock_irqsave(&hp->lock, flags);
    > > /* Check and then increment for fast path open. */
    > > if (hp->count++ > 0) {
    > > + tty_kref_get(tty);
    > > spin_unlock_irqrestore(&hp->lock, flags);
    > > hvc_kick();
    > > return 0;
    > >
    > > I don't know this code very well but we might be missing a
    > > corresponding tty_kref_put() some place ?
    >
    > See hvc_hangup:
    >
    > temp_open_count = hp->count;
    > ...
    > while(temp_open_count) {
    > --temp_open_count;
    > tty_kref_put(tty);
    > kref_put(&hp->kref, destroy_hvc_struct);
    > }

    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@samba.org>
    ---

    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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