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    SubjectRe: [PATCHv4 05/17] pps: access pps device by direct pointer
    On Sun, Nov 21, 2010 at 01:33:27AM +0300, Alexander Gordeev wrote:
    > > > - if (ret < 0) {
    > > > + if (pps == NULL) {
    > > > pr_err("cannot register PPS source \"%s\"\n", info.path);
    > > > - return ret;
    > > > + return -ENOMEM;
    > > > }
    > > > - tty->disc_data = (void *)(long)ret;
    > > > +
    > > > + spin_lock_irq(&pps_ldisc_lock);
    > > > + tty->disc_data = pps;
    > > > + spin_unlock_irq(&pps_ldisc_lock);
    > >
    > > Maybe this lock is useless... however, are we sure that before setting
    > > tty->disc_data to pps its value is null? Otherwise the dcd_change may
    > > be called with an oops! We cannot control serial port IRQ
    > > generation... :-/
    >
    > No, locking here is necessary.
    > There is only one problem this spinlock protects us from: current tty
    > code neither disables interrupts nor doesn't ensure there are no
    > references to PPS ldisc from uart_handle_dcd_change() before closing it
    > (and removing PPS source). It relies on flushing workqueue and disabling
    > input. It worked good this way until dcd_change() was added which
    > doesn't use workqueues and is called in atomic context so can't lock
    > on mutex.
    >
    > Imagine that (on SMP system) uart_handle_dcd_change() could obtain a
    > reference to ldisc and call dcd_change() until actually calling
    > pps_event(); then on another processor all the path from
    > tty_ldisc_halt() until tty_ldisc_stop() is executed. And then
    > pps_event() is called with illegal pps pointer.
    >
    > I just thought you are right that disc_data can be set not NULL by
    > another ldisc and it's a problem. But actually I just realised how to
    > fix it completely. :)
    >
    > I just have to add a spinlock to tty_struct, lock all the
    > uart_handle_dcd_change() with it and add a "barrier" between
    > tty_ldisc_halt() and tty_ldisc_close() i.e. just that:
    >
    > ...
    > spin_lock_irq();
    > spin_unlock_irq();
    > ...
    >
    > This "barrier" will ensure that there is no references to ldisc from
    > uart_handle_dcd_change(). It won't be able to obtain a new reference
    > after tty_ldisc_halt() so will become completely sane. Not disabling
    > interrupts won't be a problem because it won't be able to obtain an
    > ldisc reference until tty_ldisc_enable() which is called only after the
    > new ldisc is fully functional. If it's our ldisc than it will have both
    > dcd_change defined and a valid pps pointer. If it's not our ldisc it
    > won't have both so uart_handle_dcd_change() won't call dcd_change() at
    > all.
    >
    > I think I'll do that as a separate patch.

    Excuse me but IMHO you should solve all your problems if you do the lock into
    pps_tty_init/cleanup instead of into pps_tty_open/close.

    spin_lock_irq();
    err = tty_register_ldisc(N_PPS, &pps_ldisc_ops);
    if (err)
    pr_err("can't register PPS line discipline\n");
    else
    pr_info("PPS line discipline registered\n");
    spin_unlock_irq();

    And the same into cleanup.

    Ciao,

    Rodolfo

    --

    GNU/Linux Solutions e-mail: giometti@enneenne.com
    Linux Device Driver giometti@linux.it
    Embedded Systems phone: +39 349 2432127
    UNIX programming skype: rodolfo.giometti
    Freelance ICT Italia - Consulente ICT Italia - www.consulenti-ict.it


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