Re: [PATCH V4 1/3] livepatch: remove 'struct completion finish' from klp_patch

On Tue, Nov 02, 2021 at 04:56:10PM +0100, Petr Mladek wrote:

On Tue 2021-11-02 22:59:30, Ming Lei wrote:

quoted

The completion finish is just for waiting release of the klp_patch
object, then releases module refcnt. We can simply drop the module
refcnt in the kobject release handler of klp_patch.

This way also helps to support allocating klp_patch from heap.

IMHO, this is wrong assumption. kobject_put() might do everyting
asynchronously, see:

   kobject_put()
     kobject_release()
       INIT_DELAYED_WORK(&kobj->release, kobject_delayed_cleanup);
       schedule_delayed_work(&kobj->release, delay);

   asynchronously:

     kobject_delayed_cleanup()
      kobject_cleanup()
	__kobject_del()

OK, this is one generic kobject release vs. module unloading issue to
solve, not unique for klp module, and there should be lots of drivers
suffering from it.

quoted

Signed-off-by: Ming Lei <redacted>
---
 include/linux/livepatch.h |  1 -
 kernel/livepatch/core.c   | 12 +++---------
 2 files changed, 3 insertions(+), 10 deletions(-)

diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
index 2614247a9781..9712818997c5 100644
--- a/include/linux/livepatch.h
+++ b/include/linux/livepatch.h

@@ -170,7 +170,6 @@ struct klp_patch {
 	bool enabled;
 	bool forced;
 	struct work_struct free_work;
-	struct completion finish;
 };
 
 #define klp_for_each_object_static(patch, obj) \

diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index 335d988bd811..b967b4b0071b 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c

@@ -551,10 +551,10 @@ static int klp_add_nops(struct klp_patch *patch)
 
 static void klp_kobj_release_patch(struct kobject *kobj)
 {
-	struct klp_patch *patch;
+	struct klp_patch *patch = container_of(kobj, struct klp_patch, kobj);
 
-	patch = container_of(kobj, struct klp_patch, kobj);
-	complete(&patch->finish);
+	if (!patch->forced)
+		module_put(patch->mod);
 }
 
 static struct kobj_type klp_ktype_patch = {

@@ -678,11 +678,6 @@ static void klp_free_patch_finish(struct klp_patch *patch)
 	 * cannot get enabled again.
 	 */
 	kobject_put(&patch->kobj);
-	wait_for_completion(&patch->finish);
-
-	/* Put the module after the last access to struct klp_patch. */
-	if (!patch->forced)
-		module_put(patch->mod);

klp_free_patch_finish() does not longer wait until the release
callbacks are called.

klp_free_patch_finish() is called also in klp_enable_patch() error
path.

klp_enable_patch() is called in module_init(). For example, see
samples/livepatch/livepatch-sample.c

The module must not get removed until the release callbacks are called.
Does the module loader check the module reference counter when
module_init() fails?

Good catch, that is really one corner case, in which the kobject has to
be cleaned up before returning from mod->init(), cause there can't be
module unloading in case of mod->init() failure. 

Yeah, it should be more related with async kobject_put().

Also looks it is reasonable to add check when cleaning module loading
failure.


thanks,
Ming