On Thu, Aug 07, 2014 at 12:59:48PM -0400, chas williams - CONTRACTOR wrote:
> On Thu, 7 Aug 2014 17:17:41 +0200
> Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > Subject: atm: Fix blocking in wait loop
> > 
> > One should not call blocking primitives inside a wait loop, since both
> > require task_struct::state to sleep, so the inner will destroy the outer
> > state.
> > 
> > In this instance sigd_enq() will possible sleep for alloc_skb(), now if
> > I understand the code right, we do not actually need to call sigd_enq()
> > after the initial prepare_to_wait(), because we test the termination
> > condition before schedule() anyhow.
> > 
> > So we can simply move it up a bit and avoid the entire confusion.
> > 
> > Signed-off-by: Peter Zijlstra <peterz@infradead.org>
> > ---
> >  net/atm/svc.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/net/atm/svc.c b/net/atm/svc.c
> > index d8e5d0c2ebbc..445ac238b69b 100644
> > --- a/net/atm/svc.c
> > +++ b/net/atm/svc.c
> > @@ -297,8 +297,8 @@ static int svc_listen(struct socket *sock, int backlog)
> >  		goto out;
> >  	}
> >  	set_bit(ATM_VF_WAITING, &vcc->flags);
> > -	prepare_to_wait(sk_sleep(sk), &wait, TASK_UNINTERRUPTIBLE);
> >  	sigd_enq(vcc, as_listen, NULL, NULL, &vcc->local);
> > +	prepare_to_wait(sk_sleep(sk), &wait, TASK_UNINTERRUPTIBLE);
> >  	while (test_bit(ATM_VF_WAITING, &vcc->flags) && sigd) {
> >  		schedule();
> >  		prepare_to_wait(sk_sleep(sk), &wait, TASK_UNINTERRUPTIBLE);
> 
> 
> This isn't the only place that we queue a message for the signalling
> daemon after a prepare_to_wait() uninterruptibly so this patch would
> be incomplete as is.

I'm not sure I follow, this is the only place you do so while then going
to sleep. All other sites don't sleep while they're enqueued on the
waitqueue.

> What bothers me is the TASK_UNINTERRUPTIBLE -- I don't have a good
> reason why any of these should be sleeping uninterruptibly.

That's a whole different story, there's tons of ugly in there, but its
all ancient code.