mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/chenhuacai/linux-loongson
synced 2025-10-31 14:30:50 +00:00
e8aed68614
atomic_inc_not_zero(v) return 0 if *v = 0. use spin_lock instead of write_lock for update lock. Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
67 lines
2.2 KiB
Plaintext
67 lines
2.2 KiB
Plaintext
Reference-count design for elements of lists/arrays protected by RCU.
|
|
|
|
Reference counting on elements of lists which are protected by traditional
|
|
reader/writer spinlocks or semaphores are straightforward:
|
|
|
|
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 atomic_inc() in
|
|
search_and_reference() could potentially hold reference to an element which
|
|
has already been deleted from the list/array. Use atomic_inc_not_zero()
|
|
in this scenario as follows:
|
|
|
|
1. 2.
|
|
add() search_and_reference()
|
|
{ {
|
|
alloc_object rcu_read_lock();
|
|
... search_for_element
|
|
atomic_set(&el->rc, 1); if (!atomic_inc_not_zero(&el->rc)) {
|
|
spin_lock(&list_lock); rcu_read_unlock();
|
|
return FAIL;
|
|
add_element }
|
|
... ...
|
|
spin_unlock(&list_lock); rcu_read_unlock();
|
|
} }
|
|
3. 4.
|
|
release_referenced() delete()
|
|
{ {
|
|
... spin_lock(&list_lock);
|
|
if (atomic_dec_and_test(&el->rc)) ...
|
|
call_rcu(&el->head, el_free); delete_element
|
|
... spin_unlock(&list_lock);
|
|
} ...
|
|
if (atomic_dec_and_test(&el->rc))
|
|
call_rcu(&el->head, el_free);
|
|
...
|
|
}
|
|
|
|
Sometimes, a reference to the element needs to be obtained in the
|
|
update (write) stream. In such cases, atomic_inc_not_zero() might be
|
|
overkill, since we hold the update-side spinlock. One might instead
|
|
use atomic_inc() in such cases.
|