module: add kset_obj_exists() and use it

Commit Message

Rusty Russell April 10, 2013, 7:17 a.m. UTC

Veaceslav Falico <[e-mail]> writes:
> Add a new function, kset_obj_exists(), which is identical to
> kset_find_obj() but doesn't take a reference to the kobject
> found and only returns bool if found/not found.
>
> The main purpose would be to avoid the possible race scenario,
> when we could get the reference in between the kref_put() and
> kobject_release() functions (i.e. kref_put() already ran,
> refcount became 0, but the kobject_release() function still
> didn't run, and we try to get via kobject_get() and thus ending
> up with a released kobject).

What?  If refcount is zero, there should be no more references to it.
That's what 0 means.

If you want 0-and-then-cleanup, you need atomic_dec_and_lock().

> It can be triggered, for example,
> by running insmod/rmmod bonding in parallel, which ends up in
> a race between kset_obj_find() in mod_sysfs_init() and rmmod's
> mod_sysfs_fini()/kobject_put().

That's a bug.  We should be cleaning up sysfs before we unlike the
removed module from the list.

Because the same thing applies to ddebug info, which is also keyed by
module name.

Something like this (untested!):


> It also saves us some CPU time in several places where we don't
> need the kobject itself and only need to verify if it actually
> exists, by not taking the kref (and thus we don't need to
> kobject_put() afterwards).
>
> Signed-off-by: Veaceslav Falico <[e-mail]>
> ---
>  drivers/pci/slot.c      |    5 +----
>  include/linux/kobject.h |    1 +
>  kernel/module.c         |    5 +----
>  lib/kobject.c           |   28 ++++++++++++++++++++++++++++
>  4 files changed, 31 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/pci/slot.c b/drivers/pci/slot.c
> index ac6412f..3988d75 100644
> --- a/drivers/pci/slot.c
> +++ b/drivers/pci/slot.c
> @@ -154,11 +154,8 @@ static char *make_slot_name(const char *name)
>  	dup = 1;
>  
>  	for (;;) {
> -		struct kobject *dup_slot;
> -		dup_slot = kset_find_obj(pci_slots_kset, new_name);
> -		if (!dup_slot)
> +		if (!kset_obj_exists(pci_slots_kset, new_name))
>  			break;
> -		kobject_put(dup_slot);
>  		if (dup == max) {
>  			len++;
>  			max *= 10;
> diff --git a/include/linux/kobject.h b/include/linux/kobject.h
> index 939b112..7cde72c 100644
> --- a/include/linux/kobject.h
> +++ b/include/linux/kobject.h
> @@ -191,6 +191,7 @@ static inline struct kobj_type *get_ktype(struct kobject *kobj)
>  }
>  
>  extern struct kobject *kset_find_obj(struct kset *, const char *);
> +extern bool kset_obj_exists(struct kset *, const char *);
>  
>  /* The global /sys/kernel/ kobject for people to chain off of */
>  extern struct kobject *kernel_kobj;
> diff --git a/kernel/module.c b/kernel/module.c
> index 0925c9a..1df13a3 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -1606,7 +1606,6 @@ static void module_remove_modinfo_attrs(struct module *mod)
>  static int mod_sysfs_init(struct module *mod)
>  {
>  	int err;
> -	struct kobject *kobj;
>  
>  	if (!module_sysfs_initialized) {
>  		printk(KERN_ERR "%s: module sysfs not initialized\n",
> @@ -1615,10 +1614,8 @@ static int mod_sysfs_init(struct module *mod)
>  		goto out;
>  	}
>  
> -	kobj = kset_find_obj(module_kset, mod->name);
> -	if (kobj) {
> +	if (kset_obj_exists(module_kset, mod->name)) {
>  		printk(KERN_ERR "%s: module is already loaded\n", mod->name);
> -		kobject_put(kobj);
>  		err = -EINVAL;
>  		goto out;
>  	}
> diff --git a/lib/kobject.c b/lib/kobject.c
> index e07ee1f..b82633f 100644
> --- a/lib/kobject.c
> +++ b/lib/kobject.c
> @@ -760,6 +760,34 @@ struct kobject *kset_find_obj(struct kset *kset, const char *name)
>  	return ret;
>  }
>  
> +/**
> + * kset_obj_exists - verify if an object exists in kset
> + * @kset: kset we're looking in.
> + * @name: object's name.
> + *
> + * Lock kset via @kset->subsys, and iterate over @kset->list,
> + * looking for a matching kobject. Returns bool, found/not found.
> + * This function does not take a reference and thus doesn't
> + * guarantee that the object won't go away any time after.
> + */
> +
> +bool kset_obj_exists(struct kset *kset, const char *name)
> +{
> +	struct kobject *k;
> +
> +	spin_lock(&kset->list_lock);
> +
> +	list_for_each_entry(k, &kset->list, entry) {
> +		if (kobject_name(k) && !strcmp(kobject_name(k), name)) {
> +			spin_unlock(&kset->list_lock);
> +			return true;
> +		}
> +	}
> +
> +	spin_unlock(&kset->list_lock);
> +	return false;
> +}
> +
>  static void kset_release(struct kobject *kobj)
>  {
>  	struct kset *kset = container_of(kobj, struct kset, kobj);
> -- 
> 1.7.1
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Patch

diff --git a/kernel/module.c b/kernel/module.c
index 3c2c72d..8d4de82 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -1862,10 +1862,10 @@  static void free_module(struct module *mod)
 {
 	trace_module_free(mod);
 
-	/* Delete from various lists */
-	mutex_lock(&module_mutex);
-	stop_machine(__unlink_module, mod, NULL);
-	mutex_unlock(&module_mutex);
+	/* We leave it in list to prevent duplicate loads while we clean
+	 * up sysfs, ddebug and any other external exposure. */
+	mod->state = MODULE_STATE_UNFORMED;
+
 	mod_sysfs_teardown(mod);
 
 	/* Remove dynamic debug info */
@@ -1880,6 +1880,11 @@  static void free_module(struct module *mod)
 	/* Free any allocated parameters. */
 	destroy_params(mod->kp, mod->num_kp);
 
+	/* Now we can delete it from the lists */
+	mutex_lock(&module_mutex);
+	stop_machine(__unlink_module, mod, NULL);
+	mutex_unlock(&module_mutex);
+
 	/* This may be NULL, but that's OK */
 	unset_module_init_ro_nx(mod);
 	module_free(mod, mod->module_init);