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kref: Add kref_get_unless_zero documentation 

Document how kref_get_unless_zero should be used and how it helps
solve a typical kref / locking problem.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
This commit is contained in:
Thomas Hellstrom 2012-11-20 12:16:48 +00:00 committed by Dave Airlie
parent 384cc2f968
commit a82b8db02f
1 changed files with 88 additions and 0 deletions
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88
Documentation/kref.txt
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@ -213,3 +213,91 @@ presentation on krefs, which can be found at:
and:
http://www.kroah.com/linux/talks/ols_2004_kref_talk/
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.
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.
Thomas Hellstrom <thellstrom@vmware.com>