Messages in this thread | | | Date | Mon, 22 Nov 2010 18:01:23 +0300 | From | Alexander Gordeev <> | Subject | Re: [PATCHv4 05/17] pps: access pps device by direct pointer |
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В Sun, 21 Nov 2010 09:26:58 +0100 Rodolfo Giometti <giometti@enneenne.com> пишет:
> 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.
Sorry, I don't understand how this is supposed to help. Anyway, a new patch is underway.
-- Alexander [unhandled content-type:application/pgp-signature] | |