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SubjectRe: [PATCH v12 04/12] livepatch: Consolidate klp_free functions
On Tue, 28 Aug 2018, Petr Mladek wrote:

> The code for freeing livepatch structures is a bit scattered and tricky:
>
> + direct calls to klp_free_*_limited() and kobject_put() are
> used to release partially initialized objects
>
> + klp_free_patch() removes the patch from the public list
> and releases all objects except for patch->kobj
>
> + object_put(&patch->kobj) and the related wait_for_completion()
> are called directly outside klp_mutex; this code is duplicated;
>
> Now, we are going to remove the registration stage to simplify the API
> and the code. This would require handling more situations in
> klp_enable_patch() error paths.
>
> More importantly, we are going to add a feature called atomic replace.
> It will need to dynamically create func and object structures. We will
> want to reuse the existing init() and free() functions. This would
> create even more error path scenarios.
>
> This patch implements a more clever free functions:
>
> + checks kobj.state_initialized instead of @limit
>
> + initializes patch->list early so that the check for empty list
> always works
>
> + The action(s) that has to be done outside klp_mutex are done
> in separate klp_free_patch_end() function. It waits only
> when patch->kobj was really released via the _begin() part.
>
> Note that it is safe to put patch->kobj under klp_mutex. It calls
> the release callback only when the reference count reaches zero.
> Therefore it does not block any related sysfs operation that took
> a reference and might eventually wait for klp_mutex.

This seems to be the reason of the issue which lockdep reported. The patch
moved kobject_put(&patch->kobj) under klp_mutex. Perhaps I cannot read
kernfs code properly today, but I fail to understand why it is supposed to
be safe.

Indeed, if it is safe, lockdep report is false positive.

> Note that __klp_free_patch() is split because it will be later
> used in a _nowait() variant. Also klp_free_patch_end() makes
> sense because it will later get more complicated.

There are no _begin() and _end() functions in the patch.

> This patch does not change the existing behavior.
>
> Signed-off-by: Petr Mladek <pmladek@suse.com>
> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> Cc: Jessica Yu <jeyu@kernel.org>
> Cc: Jiri Kosina <jikos@kernel.org>
> Cc: Jason Baron <jbaron@akamai.com>
> Acked-by: Miroslav Benes <mbenes@suse.cz>

My Acked-by here is a bit premature.

> ---
> include/linux/livepatch.h | 2 ++
> kernel/livepatch/core.c | 92 +++++++++++++++++++++++++++++------------------
> 2 files changed, 59 insertions(+), 35 deletions(-)
>
> diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
> index 1163742b27c0..22e0767d64b0 100644
> --- a/include/linux/livepatch.h
> +++ b/include/linux/livepatch.h
> @@ -138,6 +138,7 @@ struct klp_object {
> * @list: list node for global list of registered patches
> * @kobj: kobject for sysfs resources
> * @enabled: the patch is enabled (but operation may be incomplete)
> + * @wait_free: wait until the patch is freed
> * @finish: for waiting till it is safe to remove the patch module
> */
> struct klp_patch {
> @@ -149,6 +150,7 @@ struct klp_patch {
> struct list_head list;
> struct kobject kobj;
> bool enabled;
> + bool wait_free;
> struct completion finish;
> };
>
> diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> index b3956cce239e..3ca404545150 100644
> --- a/kernel/livepatch/core.c
> +++ b/kernel/livepatch/core.c
> @@ -465,17 +465,15 @@ static struct kobj_type klp_ktype_func = {
> .sysfs_ops = &kobj_sysfs_ops,
> };
>
> -/*
> - * Free all functions' kobjects in the array up to some limit. When limit is
> - * NULL, all kobjects are freed.
> - */
> -static void klp_free_funcs_limited(struct klp_object *obj,
> - struct klp_func *limit)
> +static void klp_free_funcs(struct klp_object *obj)
> {
> struct klp_func *func;
>
> - for (func = obj->funcs; func->old_name && func != limit; func++)
> - kobject_put(&func->kobj);
> + klp_for_each_func(obj, func) {
> + /* Might be called from klp_init_patch() error path. */
> + if (func->kobj.state_initialized)
> + kobject_put(&func->kobj);
> + }
> }

Just for the record, it is a slightly suboptimal because now we iterate
through the whole list. We could add break to else branch, I think, but
it's not necessary.

> /* Clean up when a patched object is unloaded */
> @@ -489,26 +487,59 @@ static void klp_free_object_loaded(struct klp_object *obj)
> func->old_addr = 0;
> }
>
> -/*
> - * Free all objects' kobjects in the array up to some limit. When limit is
> - * NULL, all kobjects are freed.
> - */
> -static void klp_free_objects_limited(struct klp_patch *patch,
> - struct klp_object *limit)
> +static void klp_free_objects(struct klp_patch *patch)
> {
> struct klp_object *obj;
>
> - for (obj = patch->objs; obj->funcs && obj != limit; obj++) {
> - klp_free_funcs_limited(obj, NULL);
> - kobject_put(&obj->kobj);
> + klp_for_each_object(patch, obj) {
> + klp_free_funcs(obj);
> +
> + /* Might be called from klp_init_patch() error path. */
> + if (obj->kobj.state_initialized)
> + kobject_put(&obj->kobj);
> }
> }

Same here, of course.

Regards,
Miroslav

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