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

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Message-ID: <20080401084214.GY7279@enneenne.com>
Date:	Tue, 1 Apr 2008 10:42:14 +0200
From:	Rodolfo Giometti <giometti@...eenne.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, dwmw2@...radead.org,
	davej@...hat.com, sam@...nborg.org, greg@...ah.com,
	randy.dunlap@...cle.com
Subject: Re: [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@...eenne.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@...eenne.com
Linux Device Driver                             giometti@...dd.com
Embedded Systems                     		giometti@...ux.it
UNIX programming                     phone:     +39 349 2432127
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