mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-08-27 19:29:37 +00:00
![Mauro Carvalho Chehab](/assets/img/avatar_default.png)
Each text file under Documentation follows a different format. Some doesn't even have titles! Change its representation to follow the adopted standard, using ReST markups for it to be parseable by Sphinx: - add a title for the document and section titles; - move authorship information to the beginning and use :Author: - mark literal blocks as such and ident them if needed. Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
319 lines
8.9 KiB
Text
319 lines
8.9 KiB
Text
===================================================
|
|
Adding reference counters (krefs) to kernel objects
|
|
===================================================
|
|
|
|
:Author: Corey Minyard <minyard@acm.org>
|
|
:Author: Thomas Hellstrom <thellstrom@vmware.com>
|
|
|
|
A lot of this was lifted from Greg Kroah-Hartman's 2004 OLS paper and
|
|
presentation on krefs, which can be found at:
|
|
|
|
- http://www.kroah.com/linux/talks/ols_2004_kref_paper/Reprint-Kroah-Hartman-OLS2004.pdf
|
|
- http://www.kroah.com/linux/talks/ols_2004_kref_talk/
|
|
|
|
Introduction
|
|
============
|
|
|
|
krefs allow you to add reference counters to your objects. If you
|
|
have objects that are used in multiple places and passed around, and
|
|
you don't have refcounts, your code is almost certainly broken. If
|
|
you want refcounts, krefs are the way to go.
|
|
|
|
To use a kref, add one to your data structures like::
|
|
|
|
struct my_data
|
|
{
|
|
.
|
|
.
|
|
struct kref refcount;
|
|
.
|
|
.
|
|
};
|
|
|
|
The kref can occur anywhere within the data structure.
|
|
|
|
Initialization
|
|
==============
|
|
|
|
You must initialize the kref after you allocate it. To do this, call
|
|
kref_init as so::
|
|
|
|
struct my_data *data;
|
|
|
|
data = kmalloc(sizeof(*data), GFP_KERNEL);
|
|
if (!data)
|
|
return -ENOMEM;
|
|
kref_init(&data->refcount);
|
|
|
|
This sets the refcount in the kref to 1.
|
|
|
|
Kref rules
|
|
==========
|
|
|
|
Once you have an initialized kref, you must follow the following
|
|
rules:
|
|
|
|
1) If you make a non-temporary copy of a pointer, especially if
|
|
it can be passed to another thread of execution, you must
|
|
increment the refcount with kref_get() before passing it off::
|
|
|
|
kref_get(&data->refcount);
|
|
|
|
If you already have a valid pointer to a kref-ed structure (the
|
|
refcount cannot go to zero) you may do this without a lock.
|
|
|
|
2) When you are done with a pointer, you must call kref_put()::
|
|
|
|
kref_put(&data->refcount, data_release);
|
|
|
|
If this is the last reference to the pointer, the release
|
|
routine will be called. If the code never tries to get
|
|
a valid pointer to a kref-ed structure without already
|
|
holding a valid pointer, it is safe to do this without
|
|
a lock.
|
|
|
|
3) If the code attempts to gain a reference to a kref-ed structure
|
|
without already holding a valid pointer, it must serialize access
|
|
where a kref_put() cannot occur during the kref_get(), and the
|
|
structure must remain valid during the kref_get().
|
|
|
|
For example, if you allocate some data and then pass it to another
|
|
thread to process::
|
|
|
|
void data_release(struct kref *ref)
|
|
{
|
|
struct my_data *data = container_of(ref, struct my_data, refcount);
|
|
kfree(data);
|
|
}
|
|
|
|
void more_data_handling(void *cb_data)
|
|
{
|
|
struct my_data *data = cb_data;
|
|
.
|
|
. do stuff with data here
|
|
.
|
|
kref_put(&data->refcount, data_release);
|
|
}
|
|
|
|
int my_data_handler(void)
|
|
{
|
|
int rv = 0;
|
|
struct my_data *data;
|
|
struct task_struct *task;
|
|
data = kmalloc(sizeof(*data), GFP_KERNEL);
|
|
if (!data)
|
|
return -ENOMEM;
|
|
kref_init(&data->refcount);
|
|
|
|
kref_get(&data->refcount);
|
|
task = kthread_run(more_data_handling, data, "more_data_handling");
|
|
if (task == ERR_PTR(-ENOMEM)) {
|
|
rv = -ENOMEM;
|
|
kref_put(&data->refcount, data_release);
|
|
goto out;
|
|
}
|
|
|
|
.
|
|
. do stuff with data here
|
|
.
|
|
out:
|
|
kref_put(&data->refcount, data_release);
|
|
return rv;
|
|
}
|
|
|
|
This way, it doesn't matter what order the two threads handle the
|
|
data, the kref_put() handles knowing when the data is not referenced
|
|
any more and releasing it. The kref_get() does not require a lock,
|
|
since we already have a valid pointer that we own a refcount for. The
|
|
put needs no lock because nothing tries to get the data without
|
|
already holding a pointer.
|
|
|
|
Note that the "before" in rule 1 is very important. You should never
|
|
do something like::
|
|
|
|
task = kthread_run(more_data_handling, data, "more_data_handling");
|
|
if (task == ERR_PTR(-ENOMEM)) {
|
|
rv = -ENOMEM;
|
|
goto out;
|
|
} else
|
|
/* BAD BAD BAD - get is after the handoff */
|
|
kref_get(&data->refcount);
|
|
|
|
Don't assume you know what you are doing and use the above construct.
|
|
First of all, you may not know what you are doing. Second, you may
|
|
know what you are doing (there are some situations where locking is
|
|
involved where the above may be legal) but someone else who doesn't
|
|
know what they are doing may change the code or copy the code. It's
|
|
bad style. Don't do it.
|
|
|
|
There are some situations where you can optimize the gets and puts.
|
|
For instance, if you are done with an object and enqueuing it for
|
|
something else or passing it off to something else, there is no reason
|
|
to do a get then a put::
|
|
|
|
/* Silly extra get and put */
|
|
kref_get(&obj->ref);
|
|
enqueue(obj);
|
|
kref_put(&obj->ref, obj_cleanup);
|
|
|
|
Just do the enqueue. A comment about this is always welcome::
|
|
|
|
enqueue(obj);
|
|
/* We are done with obj, so we pass our refcount off
|
|
to the queue. DON'T TOUCH obj AFTER HERE! */
|
|
|
|
The last rule (rule 3) is the nastiest one to handle. Say, for
|
|
instance, you have a list of items that are each kref-ed, and you wish
|
|
to get the first one. You can't just pull the first item off the list
|
|
and kref_get() it. That violates rule 3 because you are not already
|
|
holding a valid pointer. You must add a mutex (or some other lock).
|
|
For instance::
|
|
|
|
static DEFINE_MUTEX(mutex);
|
|
static LIST_HEAD(q);
|
|
struct my_data
|
|
{
|
|
struct kref refcount;
|
|
struct list_head link;
|
|
};
|
|
|
|
static struct my_data *get_entry()
|
|
{
|
|
struct my_data *entry = NULL;
|
|
mutex_lock(&mutex);
|
|
if (!list_empty(&q)) {
|
|
entry = container_of(q.next, struct my_data, link);
|
|
kref_get(&entry->refcount);
|
|
}
|
|
mutex_unlock(&mutex);
|
|
return entry;
|
|
}
|
|
|
|
static void release_entry(struct kref *ref)
|
|
{
|
|
struct my_data *entry = container_of(ref, struct my_data, refcount);
|
|
|
|
list_del(&entry->link);
|
|
kfree(entry);
|
|
}
|
|
|
|
static void put_entry(struct my_data *entry)
|
|
{
|
|
mutex_lock(&mutex);
|
|
kref_put(&entry->refcount, release_entry);
|
|
mutex_unlock(&mutex);
|
|
}
|
|
|
|
The kref_put() return value is useful if you do not want to hold the
|
|
lock during the whole release operation. Say you didn't want to call
|
|
kfree() with the lock held in the example above (since it is kind of
|
|
pointless to do so). You could use kref_put() as follows::
|
|
|
|
static void release_entry(struct kref *ref)
|
|
{
|
|
/* All work is done after the return from kref_put(). */
|
|
}
|
|
|
|
static void put_entry(struct my_data *entry)
|
|
{
|
|
mutex_lock(&mutex);
|
|
if (kref_put(&entry->refcount, release_entry)) {
|
|
list_del(&entry->link);
|
|
mutex_unlock(&mutex);
|
|
kfree(entry);
|
|
} else
|
|
mutex_unlock(&mutex);
|
|
}
|
|
|
|
This is really more useful if you have to call other routines as part
|
|
of the free operations that could take a long time or might claim the
|
|
same lock. Note that doing everything in the release routine is still
|
|
preferred as it is a little neater.
|
|
|
|
The above example could also be optimized using kref_get_unless_zero() in
|
|
the following way::
|
|
|
|
static struct my_data *get_entry()
|
|
{
|
|
struct my_data *entry = NULL;
|
|
mutex_lock(&mutex);
|
|
if (!list_empty(&q)) {
|
|
entry = container_of(q.next, struct my_data, link);
|
|
if (!kref_get_unless_zero(&entry->refcount))
|
|
entry = NULL;
|
|
}
|
|
mutex_unlock(&mutex);
|
|
return entry;
|
|
}
|
|
|
|
static void release_entry(struct kref *ref)
|
|
{
|
|
struct my_data *entry = container_of(ref, struct my_data, refcount);
|
|
|
|
mutex_lock(&mutex);
|
|
list_del(&entry->link);
|
|
mutex_unlock(&mutex);
|
|
kfree(entry);
|
|
}
|
|
|
|
static void put_entry(struct my_data *entry)
|
|
{
|
|
kref_put(&entry->refcount, release_entry);
|
|
}
|
|
|
|
Which is useful to remove the mutex lock around kref_put() in put_entry(), but
|
|
it's important that kref_get_unless_zero is enclosed in the same critical
|
|
section that finds the entry in the lookup table,
|
|
otherwise kref_get_unless_zero may reference already freed memory.
|
|
Note that it is illegal to use kref_get_unless_zero without checking its
|
|
return value. If you are sure (by already having a valid pointer) that
|
|
kref_get_unless_zero() will return true, then use kref_get() instead.
|
|
|
|
Krefs and RCU
|
|
=============
|
|
|
|
The function kref_get_unless_zero also makes it possible to use rcu
|
|
locking for lookups in the above example::
|
|
|
|
struct my_data
|
|
{
|
|
struct rcu_head rhead;
|
|
.
|
|
struct kref refcount;
|
|
.
|
|
.
|
|
};
|
|
|
|
static struct my_data *get_entry_rcu()
|
|
{
|
|
struct my_data *entry = NULL;
|
|
rcu_read_lock();
|
|
if (!list_empty(&q)) {
|
|
entry = container_of(q.next, struct my_data, link);
|
|
if (!kref_get_unless_zero(&entry->refcount))
|
|
entry = NULL;
|
|
}
|
|
rcu_read_unlock();
|
|
return entry;
|
|
}
|
|
|
|
static void release_entry_rcu(struct kref *ref)
|
|
{
|
|
struct my_data *entry = container_of(ref, struct my_data, refcount);
|
|
|
|
mutex_lock(&mutex);
|
|
list_del_rcu(&entry->link);
|
|
mutex_unlock(&mutex);
|
|
kfree_rcu(entry, rhead);
|
|
}
|
|
|
|
static void put_entry(struct my_data *entry)
|
|
{
|
|
kref_put(&entry->refcount, release_entry_rcu);
|
|
}
|
|
|
|
But note that the struct kref member needs to remain in valid memory for a
|
|
rcu grace period after release_entry_rcu was called. That can be accomplished
|
|
by using kfree_rcu(entry, rhead) as done above, or by calling synchronize_rcu()
|
|
before using kfree, but note that synchronize_rcu() may sleep for a
|
|
substantial amount of time.
|