Messages in this thread | | | Date | Tue, 1 Apr 2008 11:50:37 +0200 | From | Rodolfo Giometti <> | Subject | Re: [PATCH 1/7] LinuxPPS core support. |
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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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