On Wed, Jan 03, 2007 at 04:07:34PM +0100, Blaisorblade wrote:
> > + spin_lock(&line->count_lock);
> > + if(!line->valid)
> > + goto out_unlock;
> > +
> > + err = 0;
> > + if(tty->count > 1)
> > + goto out_unlock;
> >
> > - /* The IRQ which takes this lock is not yet enabled and won't be run
> > - * before the end, so we don't need to use spin_lock_irq.*/
> > - spin_lock(&line->lock);
> > + mutex_lock(&line->open_mutex);
> > + spin_unlock(&line->count_lock);
>
> This is an obnoxious thing to do unless you specifically prove otherwise.
Didn't I?
The proof goes like this:
we only take the semaphore if tty->count == 1, in which case
we are opening the device for the first time and there can't be anyone
else looking at it, so the mutex_lock won't sleep.
However, now that you're making me think about it again, I'm wondering
about the sanity of introducing a mutex which is guaranteed not to
sleep.
This is starting to make sense, with (tty->count > 1) being the
OPENING flag:
> In the first solution, you can create a OPENING flag (via a state variable),
> and add the rule that (unlike the count) nobody but the original setter is
> allowed to change it, and that who finds it set (say a concurrent open) must
> return without touching it.
Then, I think the mutex can just be thrown away.
Jeff
--
Work email - jdike at linux dot intel dot com
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