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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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