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    SubjectRe: Killing the tty lock
    On Tue, May 08, 2012 at 11:08:29AM -0700, Yinghai Lu wrote:
    > On Wed, May 2, 2012 at 1:36 PM, Greg KH <gregkh@linuxfoundation.org> wrote:
    > > On Wed, May 02, 2012 at 11:45:15AM +0100, Alan Cox wrote:
    > >> > It's mostly pretty "sane", but what is this:
    > >> >
    > >> > > +/*
    > >> > > + * Getting the big tty mutex for a pair of ttys with lock ordering
    > >> > > + * On a non pty/tty pair tty2 can be NULL which is just fine.
    > >> > > + */
    > >> > > +void __lockfunc tty_lock_pair(struct tty_struct *tty,
    > >> > > +                                 struct tty_struct *tty2)
    > >> > > +{
    > >> > > + if (tty < tty2) {
    > >> > > +         tty_lock(tty);
    > >> > > +         tty_lock(tty2);
    > >> > > + } else {
    > >> > > +         if (tty2 && tty2 != tty)
    > >> > > +                 tty_lock(tty2);
    > >> > > +         tty_lock(tty);
    > >> > > + }
    > >> > > +}
    > >> > > +EXPORT_SYMBOL(tty_lock_pair);
    > >> > > +
    > >> > > +void __lockfunc tty_unlock_pair(struct tty_struct *tty,
    > >> > > +                                         struct tty_struct *tty2)
    > >> > > +{
    > >> > > + tty_unlock(tty);
    > >> > > + if (tty2 && tty2 != tty)
    > >> > > +         tty_unlock(tty2);
    > >> > > +}
    > >> > > +EXPORT_SYMBOL(tty_unlock_pair);
    > >> >
    > >> > for?
    > >>
    > >> We need to take locks on a pair of tty devices at the same time in some
    > >> cases (pty/tty pairs).
    > >
    > > Ok.
    > >
    > >> > And what's with the comparing of pointers as "<"?  How portable is that
    > >> > really, and how are we supposed to control the memory location of these
    > >> > structures?
    > >>
    > >> You don't need to. The point is that we must lock any arbitrary pair of
    > >> tty structs in a defined order. Pointer comparisons work just fine for
    > >> this. The fs layer uses similar logic for inode locking. We only care
    > >> that for any given pair of objects the lock ordering is consistent.
    > >
    > > Ah, ok, that makes more sense, sorry, I didn't understand that.
    >
    > looks like some patches from Alan in your tty-next cause dead look...

    Yes, Alan is currently working on it...
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