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SubjectRe: [PATCH 1/7] LinuxPPS core support.
On Thu, Mar 27, 2008 at 08:25:31PM -0700, Andrew Morton wrote:
> On Tue, 25 Mar 2008 15:44:00 +0100 Rodolfo Giometti <giometti@enneenne.com> wrote:
> > >
> > > As it stands, there might be deadlocks such as when a process which itself
> > > holds a ref on the pps_device (with an open fd?) calls
> > > pps_unregister_source.
> >
> > I can add a wait_event_interruptible in order to allow userland to
> > continue by receiving a signal. It could be acceptable?
>
> There should be no need to "wait" for anything. When the final reference
> to an object is released, that object is cleaned up. Just like we do for
> inodes, dentries, pages, files, and 100 other kernel objects.
>
> The need to wait for something else to go away is a big red flag with
> "busted refcounting" written on it.
>
> > > Also, we need to take care that all processes which were waiting in
> > > pps_unregister_source() get to finish their cleanup before we permit rmmod
> > > to proceed. Is that handled somewhere?
> >
> > I don't understand the problem... this code as been added in order to
> > avoid the case where a pps_event() is called while a process executes
> > the pps_unregister_source(). If more processes try to execute this
> > code the first which enters will execute idr_remove() which prevents
> > another process to reach the wait_event()... is that wrong? =:-o
>
> I was asking you!
>
> We should get the reference counting and object lifetimes sorted out first.
> There should be no "wait for <object> to be released" code. Once that is
> in place, things like rmmod will also sort themselves out: it just won't be
> possible to remove the module while there are live references to objects.

The problem is related to serial and parallel clients.

The PPS source related to a serial port (or a parallel one) uses the
serial (or parallel) IRQ to get PPS timestamps and it could be
possible that a process tries to close the PPS source while another
CPU is runnig the serial IRQ, so I cannot remove the PPS object until
the IRQ handler is finished its job on the PPS object.

For clients (currently none :) which define their own IRQ handler for
PPS timestamps managing the problem doesn't arise at all.

Ciao,

Rodolfo

--

GNU/Linux Solutions e-mail: giometti@enneenne.com
Linux Device Driver giometti@gnudd.com
Embedded Systems giometti@linux.it
UNIX programming phone: +39 349 2432127


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