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    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


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