Date: Thu, 1 Oct 2009 20:19:20 +0200
From: Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= 
	<u.kleine-koenig@pengutronix.de>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>,
       linux-kernel@vger.kernel.org, Sam Ravnborg <sam@ravnborg.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       Rusty Russell <rusty@rustcorp.com.au>
Subject: Re: [PATCH 31/34] move virtrng_remove to .devexit.text
Message-ID: <20091001181920.GC27288@pengutronix.de>
References: <20091001082607.GA2181@pengutronix.de> <1254385718-14254-31-git-send-email-u.kleine-koenig@pengutronix.de> <200910011044.48293.borntraeger@de.ibm.com> <20091001091218.GE5718@redhat.com> <20091001093106.GA6386@pengutronix.de> <20091001094518.GI5718@redhat.com> <20091001101727.GC6386@pengutronix.de> <20091001123213.GL5718@redhat.com> <20091001174116.GA27288@pengutronix.de> <20091001180500.GA11984@redhat.com>
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Hello Michael,

On Thu, Oct 01, 2009 at 08:05:00PM +0200, Michael S. Tsirkin wrote:
> On Thu, Oct 01, 2009 at 07:41:16PM +0200, Uwe Kleine-K?nig wrote:
> > Hello Michael,
> > 
> > > > But note it's not an error in general to use a .text function as remove
> > > > callback.  E.g. take drivers/gpio/twl4030-gpio.c.  gpio_twl4030_remove
> > > > is used in gpio_twl4030_probe which is defined using __devinit.  So
> > > > using __devexit for gpio_twl4030_remove is wrong.  (So there is a bug,
> > > > as gpio_twl4030_remove uses __devexit.)  I didn't try, but as far as I
> > > > understand this will result in a compile error if the driver is built-in
> > > > with HOTPLUG=n.
> > > 
> > > Wait a second.
> > > As far as I understand, __devexit makes it possible to remove code if
> > > hotplug is off.
> > right.
> >  
> > > At least for static functions, it's enough to mark their only use
> > > as _devexit_p, and compiler will remove the text as it's unused.
> > > 
> > > Isn't that right?
> > hmm, I don't know.  I'll try, one moment.  OK, you're right.  The
> > function is discarded with a compiler warning.
> >  
> > > If so, what, again, was the motivation for the patches that added
> > > __devexit to functions that were already used with __devexit_p?
> > I thought it saves some memory, but as it looks now it only fixes a
> > compiler warning.
> 
> We can redefine __devexit_p(x) to something like
> #define __devexit_p(x) ((typeof(x) *)NULL)
> and this will shut down the warning without need to fix the code.
Mmmmh, don't know.  A definitive advantage is that there is only a
single point in the code that defines if a function is discarded or not.
Nothing that needs to be consistent.
For me it feels somehow wrong anyhow, but that might be only because I'm
used to the current model.
 
> > Note there are two types of errors fixed in this series.  One is:
> > 
> > 	-static int func(void arg)
> > 	+static int __devexit func(void arg)
> > 
> > if the only usage of func() is wraped by __devexit_p.  This is (as
> > seen above) not that critical, there is only a warning fixed.
> > 
> > The other type results in a build failure:
> > 
> > 	-remove = __devexit_p(another_func),
> > 	+remove = __exit_p(another_func),
> > 
> > with another_func being defined using __exit.  In the case 
> > defined(MODULE) && defined(CONFIG_HOTPLUG) another_func is discarded,
> > but __devexit_p(another_func) evaluates to another_func and thus the
> > module doesn't link.
> 
> Yes, calling __exit function from non- __exit is always a bug.
> I think there's a make flag to warn about this, not sure why it's
> not the default.
I think it was disabled because there were too many warnings :-)

Best regards
Uwe

-- 
Pengutronix e.K.                              | Uwe Kleine-K?nig            |
Industrial Linux Solutions                    | http://www.pengutronix.de/  |
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