Re: [PATCH v2 2/8] IB/core: Replace semaphore sm_sem with completion

[Jason Gunthorpe]

On Tue, Oct 25, 2016 at 08:38:21PM +0530, Binoy Jayan wrote:
> On 25 October 2016 at 18:13, Jack Wang <xjtuwjp@xxxxxxxxx> wrote:
> > Hi Binoy,
> >
> > snip
> >>
> >>         port->ib_dev   = device;
> >>         port->port_num = port_num;
> >> -       sema_init(&port->sm_sem, 1);
> >> +       init_completion(&port->sm_comp);
> >> +       complete(&port->sm_comp);
> >
> > Why complete here?
> >
> >>         mutex_init(&port->file_mutex);
> >>         INIT_LIST_HEAD(&port->file_list);
> >>
> > KR,
> > Jinpu
> 
> 
> Hi Jack,
> 
> ib_umad_sm_open() calls wait_for_completion_interruptible() which
> comes before ib_umad_sm_close() that calls complete(). In the
> initial open() there will not be anybody to signal the completion,
> so the complete is called to mark the initial state.  I am not sure
> if this is the right way to do it, though.

Using a completion to model exclusive ownership seems convoluted to
me - is that a thing now? What about an atomic?

open:

while (true) {
   wait_event_interruptible(priv->queue,test_bit(CLAIMED_BIT, &priv->flags));
   if (!test_and_set_bit(CLAIMED_BIT, &priv->flags))
        break;
}

close():

   clear_bit(CLAIMED_BIT, &priv->flags)
   wake_up(&priv->queue);

??

Jason