lkml.org 
[lkml]   [2008]   [Apr]   [1]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 1/7] LinuxPPS core support.
On Tue, Apr 01, 2008 at 01:55:55AM -0700, Andrew Morton wrote:
> On Tue, 1 Apr 2008 10:42:14 +0200 Rodolfo Giometti <giometti@enneenne.com> wrote:
>
> > 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.
>
> This can all be handled with suitable locking and refcounting. The device
> which is delivering PPS interrupts has a reference on the PPS data
> structures. If userspace has PPS open then it also has a reference.
>
> The thread of control which releases the last reference to the PPS data
> structures also frees them all up. This may require a schedule_work() if
> we need to support release-from-interrupt (as it appears that we do), but
> that's OK - we just need to be able to make the PPS data structures
> ineligible for new lookups while the schedule_work() is pending.
>
> There should be no need for any thread of control to wait for any other thread
> of control to do anything. Get the refcounting right and everything
> can be done synchronously.

So, if I well understand your suggestion, I should manage the object
clean-up into pps_cdev_release() when pps->usage reaches 0, so the
pps_unregister_source() can do only the following two steps:

pps_unregister_cdev(pps);
kfree(pps);

Is that right?

Also, can you please suggest me an example (URL or filename) about
schedule_work() usage in case of release-from-interrupt?

Thanks,

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


\
 
 \ /
  Last update: 2008-04-01 11:53    [from the cache]
©2003-2014 Jasper Spaans. Advertise on this site