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    SubjectRe: [RFC] Raceless module interface
    In message <E17pg3H-0007pb-00@starship> you write:
    > On Friday 13 September 2002 03:30, Rusty Russell wrote:
    > > In message <Pine.LNX.4.44.0209121520300.28515-100000@serv> you write:
    > > > The usecount is optional, the only important question a module must be
    > > > able to answer is: Are there any objects/references belonging to the
    > > > module? It's a simple yes/no question. If a module can't answer that, it
    > > > likely has more problem than just module unloading.
    > >
    > > Ah, we're assuming you insert synchronize_kernel() between the call
    > > to stop and the call to exit?
    > >
    > > In which case *why* do you check the use count *inside* exit_affs_fs?
    > > Why not get exit_module() to do "if (mod->usecount() != 0) return
    > > -EBUSY; else mod->exit();"?
    > Because mod->usecount may be a totally inadequate way of determining
    > if a module is busy. How does it work for LSM, for example?

    As established previously: unless the hooks do it for you, you keep
    track of it yourself before you sleep.

    > > There's the other issue of symmetry. If you allocate memory, in
    > > start, do you clean it up in stop or exit?
    > Actually, I'm going to press you on why you think you even need a
    > two stage stop. I know you have your reasons, but I doubt any of
    > the effects you aim at cannot be achieved with a single stage
    > stop/exit. Could you please summarize the argument in favor of the
    > two stage stop?

    Of course you can simulate a two-stage within a single-stage, of course,
    by doing int single_stage(void) { stage_one(); stage_two(); }, so
    "need" is a bit strong.

    Basically, you can't do stage two until you know noone is using the
    module, but you can do stage one at any time. Basically stage 1
    ensures that the refcount never *increases* by removing all external
    references to the module (ie. deregister, etc). Stage 2 then knows
    that if the refcnt is zero, there's no race and it can destroy they
    module data structures.

    The synchronize_kernel() covers those "I was about to bump the module
    count!" races, as long as noone explicitly sleeps before mod_inc, or
    after mod_dec.

    > > Similarly for other
    > > resources: you call mod->exit() every time start fails, so that is
    > > supposed to check that mod->start() succeeded?
    > He does? That's not right. ->start should clean up after itself if
    > it fails, like any other good Linux citizen.

    From my reading, yes.

    > > Of course, separating start into "init" and "start" allows you to
    > > solve the half-initialized problem as well as clarify the rules.
    > I doubt it gives any new capability at all.

    Since I explained what it does for you at the kernel summit, you
    obviously aren't listening. If you split registration interfaces into
    reserve (can fail) and use (can't fail), then you do:

    int my_module_init(void)
    int ret;
    ret = reserve_foo();
    if (ret != 0)
    return ret;
    ret = reserve_bar();
    if (ret != 0)
    return ret;

    void my_module_start(void)

    Note the symmetry here with the exit case: noone can actually use the
    module until my_module_start is called, so even if the reserve_bar()
    fails, we're safe.

    > The same with the entrenched separation at the user level between
    > create and init module: what does it give you that an error exit
    > from a single create/init would not?

    That's done for entirely different reasons, as the userspace linker
    needs to know the address of the module.

    > Sure, I know it's not going to change, but I'd like to know what the
    > thinking was, and especially, if there's a non-bogus reason, I'd
    > like to know it.

    You should probably start playing with my code if you're really

    Anyone who quotes me in their sig is an idiot. -- Rusty Russell.
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