Re: [RFC] Migrating net/sched to new module interface

[Kronos]

Il Tue, Jan 14, 2003 at 01:32:56AM +0300, Alexey N. Kuznetsov ha scritto: 
> Which does not matter at all, because the hole
> 
> void cleanup_module(void)
> {
> <an instance is cloned here>
>         unregister_qdisc(&cbq_qdisc_ops);
> }
> 
> remained in any case, be it under some preemptive, nonprepemtive lock or
> not under a lock at all.

Hmm, this can't happen now. The try_module_get() will fail if the module
is going away and so qdisc_create won't create a new qdisc.

> BTW, Rusty, a question... I do not understand, what is purpose of this
> "new" module stuff at all?

The  idea  is  to  use   try_module_get()  before  using  any  interface
registered by the module. Every module has  a state associated to it: if
the module  has been  loaded but  it is  still in  its init  section its
state  will be  MODULE_STATE_COMING  (and  try_module_get() will  fail);
if  the  module is  in  process  of being  unloaded  its  state will  be
MODULE_STATE_LIVE (and try_module_get() will  fail); otherwise the state
will be MODULE_STATE_LIVE (and try_module_get() will success).

If we use try_module_get() before using the module and module_put() when
we are done everything will work:

int create_foo(foo **new) {
        if (try_module_get(foo->owner) == 0)
                /* Module is being unloaded, it's not safe to use it */
                return -ENOSYS;

        *new = foo->new_foo();
}

void destroy_foo(foo *foo) {
        foo->finalize();

        module_put(foo->owner);
}

This is  what I've  done for the  packet scheduler:  try_module_get() is
called *before*  cloning the qdisc  and if it  fails the qdisc  won't be
cloned. module_put()  is called  in qdisc_destroy(),  after ->destroy(),
when we are sure that nobody will ever use a reference to the qdisc.

Luca
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Non capisco tutta questa eccitazione per il Multitasking: 
io sono anni che leggo in bagno.