Re: [PATCH 1/7] LinuxPPS core support.

[Rodolfo Giometti]

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@xxxxxxxxxxxx> 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@xxxxxxxxxxxx> 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

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