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    SubjectRe: [RFC PATCH 0/6] module, kbuild: Faster boot with custom kernel.
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    On Friday 20 February 2009 11:28:55 Rusty Russell wrote:
    > + /* Unlink carefully: kallsyms could be walking list. */
    > + __list_del(&mod->list);
    > + synchronize_sched();

    That should be list_del_rcu() of course.

    Rusty.

    module: don't use stop_machine on module load

    Kay Sievers <kay.sievers@vrfy.org> discovered that boot times are slowed
    by about half a second because all the stop_machine_create() calls,
    and he only probes about 40 modules (I have 125 loaded on this laptop).

    We only do stop_machine_create() so we can unlink the module if
    something goes wrong, but it's overkill (and buggy anyway: if
    stop_machine_create() fails we still call stop_machine_destroy()).

    Since we are only protecting against kallsyms (esp. oops) walking the
    list, synchronize_sched() is sufficient (synchronize_rcu() is probably
    sufficient, but we're not in a hurry).

    Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
    ---
    kernel/module.c | 12 +++---------
    1 file changed, 3 insertions(+), 9 deletions(-)
    diff --git a/kernel/module.c b/kernel/module.c
    --- a/kernel/module.c
    +++ b/kernel/module.c
    @@ -1881,12 +1881,6 @@ static noinline struct module *load_modu
    if (len > 64 * 1024 * 1024 || (hdr = vmalloc(len)) == NULL)
    return ERR_PTR(-ENOMEM);

    - /* Create stop_machine threads since the error path relies on
    - * a non-failing stop_machine call. */
    - err = stop_machine_create();
    - if (err)
    - goto free_hdr;
    -
    if (copy_from_user(hdr, umod, len) != 0) {
    err = -EFAULT;
    goto free_hdr;
    @@ -2271,12 +2265,13 @@ static noinline struct module *load_modu
    /* Get rid of temporary copy */
    vfree(hdr);

    - stop_machine_destroy();
    /* Done! */
    return mod;

    unlink:
    - stop_machine(__unlink_module, mod, NULL);
    + /* Unlink carefully: kallsyms could be walking list. */
    + list_del_rcu(&mod->list);
    + synchronize_sched();
    module_arch_cleanup(mod);
    cleanup:
    kobject_del(&mod->mkobj.kobj);
    @@ -2297,7 +2292,6 @@ static noinline struct module *load_modu
    kfree(args);
    free_hdr:
    vfree(hdr);
    - stop_machine_destroy();
    return ERR_PTR(err);

    truncated:

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