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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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