Re: [Xenomai-core] [pipe.c] hairy synchronization -> "flush the output queue upon closure"

[Dmitry Adamushko]

Philippe Gerum <[EMAIL PROTECTED]> wrote on 18.11.2005 11:14:26:

> > ...
> >
> > it looks like we can't make the whole xnpipe_release() atomic because of
> > PREEMPT_RT + wake_up_interruptible_all() things, right? Or no.
>
> You must _never_ _ever_ reschedule with the nucleus lock held; this
> is a major cause of
> jittery I recently stumbled upon that was induced by
> xnpipe_read_wait() at that time. So
> indeed, xnpipe_release() cannot be made atomic this way under a
> fully preemptible kernel.

Yep.

Now keeping in mind the observation I have made yesterday, it looks like, in fact, there is no need in wake_up_*(readers) call in file_operations::release(). There is nobody to be woken up at the time when release() is called:

1) The reference counter of "file" object is 0, i.e. there are no readers since read() increases a counter before getting blocked.

2) noone else can use anew that "file" object since close() does the following:

filp = files->fd[fd];
if (!filp)
goto out_unlock;
files->fd[fd] = NULL; <--- it's invalid from now on

so it's not possible that some new readers may occur when a counter == 0.

Hm.. but we still have fasync_helper(-1,file,0,&state->asyncq); which is about sending a signal and that's perfectly valid (a file::counter is not involved here). And that call may lead to re-scheduling (linux re-scheduling of course) so we can't put it in a blocked section.

So the best way I see is to have something like():

xnpipe_drop_user_conn()
{
xnlock_get_irqsave(&nklock,s);

while ((holder = getq(&state->outq)) != NULL)
               state->output_handler(minor,link2mh(holder),-EPIPE,state->cookie);
           }

       while ((holder = getq(&state->inq)) != NULL)
           {
           if (state->input_handler != NULL)
               state->input_handler(minor,link2mh(holder),-EPIPE,state->cookie);
           else if (state->alloc_handler == NULL)
               xnfree(link2mh(holder));
           }

__clrbits(state->status,XNPIPE_USER_CONN);

xnlock_put_irqrestore(&nklock,s);
}

and call it everywhere instead of clrbits(state->status,XNPIPE_USER_CONN);

This way we may be sure there are no pending messages left.


> --
>
> Philippe.

---

Dmitry