[PATCH] kobject: introduce kobj_completion

[PATCH] kobject: introduce kobj_completion

[Jeff Mahoney]

A common way to handle kobject lifetimes in embedded in objects with
different lifetime rules is to pair the kobject with a struct completion.

This introduces a kobj_completion structure that can be used in place
of the pairing, along with several convenience functions for
initialization, release, and put-and-wait.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
---
 include/linux/kobj_completion.h |   18 ++++++++++++++
 lib/kobject.c                   |   50 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 68 insertions(+)

--- /dev/null	1970-01-01 00:00:00.000000000 +0000
+++ b/include/linux/kobj_completion.h	2013-09-10 12:58:03.530554144 -0400
@@ -0,0 +1,18 @@
+#ifndef _KOBJ_COMPLETION_H_
+#define _KOBJ_COMPLETION_H_
+
+#include <linux/kobject.h>
+#include <linux/completion.h>
+
+struct kobj_completion {
+	struct kobject kc_kobj;
+	struct completion kc_unregister;
+};
+
+#define kobj_to_kobj_completion(kobj) \
+	container_of(kobj, struct kobj_completion, kc_kobj)
+
+void kobj_completion_init(struct kobj_completion *kc, struct kobj_type *ktype);
+void kobj_completion_release(struct kobject *kobj);
+void kobj_completion_del_and_wait(struct kobj_completion *kc);
+#endif /* _KOBJ_COMPLETION_H_ */
--- a/lib/kobject.c	2013-09-10 12:57:54.198666613 -0400
+++ b/lib/kobject.c	2013-09-10 15:22:15.122724060 -0400
@@ -13,6 +13,7 @@
  */
 
 #include <linux/kobject.h>
+#include <linux/kobj_completion.h>
 #include <linux/string.h>
 #include <linux/export.h>
 #include <linux/stat.h>
@@ -711,6 +712,55 @@ const struct sysfs_ops kobj_sysfs_ops =
 };
 
 /**
+ * kobj_completion_init - initialize a kobj_completion object.
+ * @kc: kobj_completion
+ * @ktype: type of kobject to initialize
+ *
+ * kobj_completion structures can be embedded within structures with different
+ * lifetime rules.  During the release of the enclosing object, we can
+ * wait on the release of the kobject so that we don't free it while it's
+ * still busy.
+ */
+void kobj_completion_init(struct kobj_completion *kc, struct kobj_type *ktype)
+{
+	init_completion(&kc->kc_unregister);
+	kobject_init(&kc->kc_kobj, ktype);
+}
+EXPORT_SYMBOL_GPL(kobj_completion_init);
+
+/**
+ * kobj_completion_release - release a kobj_completion object
+ * @kobj: kobject embedded in kobj_completion
+ *
+ * Used with kobject_release to notify waiters that the kobject has been
+ * released.
+ */
+void kobj_completion_release(struct kobject *kobj)
+{
+	struct kobj_completion *kc = kobj_to_kobj_completion(kobj);
+	complete(&kc->kc_unregister);
+}
+EXPORT_SYMBOL_GPL(kobj_completion_release);
+
+/**
+ * kobj_completion_del_and_wait - release the kobject and wait for it
+ * @kc: kobj_completion object to release
+ *
+ * Delete the kobject from sysfs and drop the reference count.  Then wait
+ * until any other outstanding references are also dropped.  This routine
+ * is only necessary once other references may have been taken on the
+ * kobject.  Typically this happens when the kobject has been published
+ * to sysfs via kobject_add.
+ */
+void kobj_completion_del_and_wait(struct kobj_completion *kc)
+{
+	kobject_del(&kc->kc_kobj);
+	kobject_put(&kc->kc_kobj);
+	wait_for_completion(&kc->kc_unregister);
+}
+EXPORT_SYMBOL_GPL(kobj_completion_del_and_wait);
+
+/**
  * kset_register - initialize and add a kset.
  * @k: kset.
  */

Re: [PATCH] kobject: introduce kobj_completion

[Greg KH]

On Wed, Sep 11, 2013 at 01:00:30PM -0400, Jeff Mahoney wrote:
> A common way to handle kobject lifetimes in embedded in objects with
> different lifetime rules is to pair the kobject with a struct completion.
> 
> This introduces a kobj_completion structure that can be used in place
> of the pairing, along with several convenience functions for
> initialization, release, and put-and-wait.
> 
> Signed-off-by: Jeff Mahoney <jeffm@suse.com>
> ---
>  include/linux/kobj_completion.h |   18 ++++++++++++++

Does this really need a new .h file, or can we just cram it into
kobject.h?  It's only 13 more lines to add to that file.

thanks,

greg k-h

Re: [PATCH] kobject: introduce kobj_completion

[Al Viro]

On Wed, Sep 11, 2013 at 01:00:30PM -0400, Jeff Mahoney wrote:
> A common way to handle kobject lifetimes in embedded in objects with
> different lifetime rules is to pair the kobject with a struct completion.
> 
> This introduces a kobj_completion structure that can be used in place
> of the pairing, along with several convenience functions for
> initialization, release, and put-and-wait.

That's very easy to misuse, leading to deadlocks...

Re: [PATCH] kobject: introduce kobj_completion

[Jeff Mahoney]

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On 9/11/13 1:28 PM, Al Viro wrote:
> On Wed, Sep 11, 2013 at 01:00:30PM -0400, Jeff Mahoney wrote:
>> A common way to handle kobject lifetimes in embedded in objects with
>> different lifetime rules is to pair the kobject with a struct completion.
>>
>> This introduces a kobj_completion structure that can be used in place
>> of the pairing, along with several convenience functions for
>> initialization, release, and put-and-wait.
> 
> That's very easy to misuse, leading to deadlocks...

Misuse how? If you're using it any time other than shutting down an
object, you're shooting yourself in the foot.

-Jeff

-- 
Jeff Mahoney
SUSE Labs


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Re: [PATCH] kobject: introduce kobj_completion

[Jeff Mahoney]

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On 9/11/13 1:15 PM, Greg KH wrote:
> On Wed, Sep 11, 2013 at 01:00:30PM -0400, Jeff Mahoney wrote:
>> A common way to handle kobject lifetimes in embedded in objects with
>> different lifetime rules is to pair the kobject with a struct completion.
>>
>> This introduces a kobj_completion structure that can be used in place
>> of the pairing, along with several convenience functions for
>> initialization, release, and put-and-wait.
>>
>> Signed-off-by: Jeff Mahoney <jeffm@suse.com>
>> ---
>>  include/linux/kobj_completion.h |   18 ++++++++++++++
> 
> Does this really need a new .h file, or can we just cram it into
> kobject.h?  It's only 13 more lines to add to that file.

It doesn't need to be, but it avoids everywhere that already uses a
kobject having to include <linux/completion.h> indirectly.

-Jeff

-- 
Jeff Mahoney
SUSE Labs


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