c0dfb29051
Adds a set of primitives to do reference counting for objects that are looked up without locks using RCU. Signed-off-by: Ravikiran Thirumalai <kiran_th@gmail.com> Signed-off-by: Dipankar Sarma <dipankar@in.ibm.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
75 lines
2.3 KiB
Plaintext
75 lines
2.3 KiB
Plaintext
Refcounter framework for elements of lists/arrays protected by
|
|
RCU.
|
|
|
|
Refcounting on elements of lists which are protected by traditional
|
|
reader/writer spinlocks or semaphores are straight forward as in:
|
|
|
|
1. 2.
|
|
add() search_and_reference()
|
|
{ {
|
|
alloc_object read_lock(&list_lock);
|
|
... search_for_element
|
|
atomic_set(&el->rc, 1); atomic_inc(&el->rc);
|
|
write_lock(&list_lock); ...
|
|
add_element read_unlock(&list_lock);
|
|
... ...
|
|
write_unlock(&list_lock); }
|
|
}
|
|
|
|
3. 4.
|
|
release_referenced() delete()
|
|
{ {
|
|
... write_lock(&list_lock);
|
|
atomic_dec(&el->rc, relfunc) ...
|
|
... delete_element
|
|
} write_unlock(&list_lock);
|
|
...
|
|
if (atomic_dec_and_test(&el->rc))
|
|
kfree(el);
|
|
...
|
|
}
|
|
|
|
If this list/array is made lock free using rcu as in changing the
|
|
write_lock in add() and delete() to spin_lock and changing read_lock
|
|
in search_and_reference to rcu_read_lock(), the rcuref_get in
|
|
search_and_reference could potentially hold reference to an element which
|
|
has already been deleted from the list/array. rcuref_lf_get_rcu takes
|
|
care of this scenario. search_and_reference should look as;
|
|
|
|
1. 2.
|
|
add() search_and_reference()
|
|
{ {
|
|
alloc_object rcu_read_lock();
|
|
... search_for_element
|
|
atomic_set(&el->rc, 1); if (rcuref_inc_lf(&el->rc)) {
|
|
write_lock(&list_lock); rcu_read_unlock();
|
|
return FAIL;
|
|
add_element }
|
|
... ...
|
|
write_unlock(&list_lock); rcu_read_unlock();
|
|
} }
|
|
3. 4.
|
|
release_referenced() delete()
|
|
{ {
|
|
... write_lock(&list_lock);
|
|
rcuref_dec(&el->rc, relfunc) ...
|
|
... delete_element
|
|
} write_unlock(&list_lock);
|
|
...
|
|
if (rcuref_dec_and_test(&el->rc))
|
|
call_rcu(&el->head, el_free);
|
|
...
|
|
}
|
|
|
|
Sometimes, reference to the element need to be obtained in the
|
|
update (write) stream. In such cases, rcuref_inc_lf might be an overkill
|
|
since the spinlock serialising list updates are held. rcuref_inc
|
|
is to be used in such cases.
|
|
For arches which do not have cmpxchg rcuref_inc_lf
|
|
api uses a hashed spinlock implementation and the same hashed spinlock
|
|
is acquired in all rcuref_xxx primitives to preserve atomicity.
|
|
Note: Use rcuref_inc api only if you need to use rcuref_inc_lf on the
|
|
refcounter atleast at one place. Mixing rcuref_inc and atomic_xxx api
|
|
might lead to races. rcuref_inc_lf() must be used in lockfree
|
|
RCU critical sections only.
|