linux-kernel - Re: cpufreq: mediatek: allow modular build

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Message-ID: <20150911101341.GN21084@n2100.arm.linux.org.uk>
Date:	Fri, 11 Sep 2015 11:13:41 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Arnd Bergmann <arnd@...db.de>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Pi-Cheng Chen <pi-cheng.chen@...aro.org>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: cpufreq: mediatek: allow modular build

On Fri, Sep 11, 2015 at 03:39:16PM +0530, Viresh Kumar wrote:
> On 11-09-15, 10:58, Russell King - ARM Linux wrote:
> > Not module "unplug" (I've never heard it called that before).  A module
> > with a module_init() but no module_exit() can only be added to a running
> > kernel, and never removed.  That's intentional behaviour.
> 
> Ah, I see. I wasn't sure that a module with no module_exit() can't be
> removed.

Yes, but it's not that simple.

A module with no module_init() and no module_exit() is what's called a
library module, which can be inserted, and later removed provided no
other module depends on anything the library module exports.

A module with a module_init() but no module_exit() is one which can be
inserted, but never removed.

A module with a module_init() and a module_exit() can be inserted, and
later removed in much the same way as the library module mentioned above.

See kernel/module.c:

        /* If it has an init func, it must have an exit func to unload */
        if (mod->init && !mod->exit) {
                forced = try_force_unload(flags);
                if (!forced) {
                        /* This module can't be removed */
                        ret = -EBUSY;
                        goto out;
                }
        }

and

        if (mod->init != NULL && mod->exit == NULL) {
                printed_something = 1;
                seq_puts(m, "[permanent],");
        }


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