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    SubjectRe: [PATCH 4/7] hvc_console: Fix race between hvc_close and hvc_remove
    On (Tue) Apr 06 2010 [21:42:38], Anton Blanchard wrote:
    >
    > 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.

    OK, makes sense and since it works for you,

    Acked-by: Amit Shah <amit.shah@redhat.com>

    > 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>


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