linux-kernel - Re: Killing the tty lock

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Date:	Wed, 9 May 2012 08:36:11 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, linux-kernel@...r.kernel.org
Subject: Re: 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@...uxfoundation.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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