Based on kernel version 2.6.25. Page generated on 2008-04-18 21:22 EST.
1 Reference-count design for elements of lists/arrays protected by RCU. 2 3 Reference counting on elements of lists which are protected by traditional 4 reader/writer spinlocks or semaphores are straightforward: 5 6 1. 2. 7 add() search_and_reference() 8 { { 9 alloc_object read_lock(&list_lock); 10 ... search_for_element 11 atomic_set(&el->rc, 1); atomic_inc(&el->rc); 12 write_lock(&list_lock); ... 13 add_element read_unlock(&list_lock); 14 ... ... 15 write_unlock(&list_lock); } 16 } 17 18 3. 4. 19 release_referenced() delete() 20 { { 21 ... write_lock(&list_lock); 22 atomic_dec(&el->rc, relfunc) ... 23 ... delete_element 24 } write_unlock(&list_lock); 25 ... 26 if (atomic_dec_and_test(&el->rc)) 27 kfree(el); 28 ... 29 } 30 31 If this list/array is made lock free using RCU as in changing the 32 write_lock() in add() and delete() to spin_lock and changing read_lock 33 in search_and_reference to rcu_read_lock(), the atomic_get in 34 search_and_reference could potentially hold reference to an element which 35 has already been deleted from the list/array. Use atomic_inc_not_zero() 36 in this scenario as follows: 37 38 1. 2. 39 add() search_and_reference() 40 { { 41 alloc_object rcu_read_lock(); 42 ... search_for_element 43 atomic_set(&el->rc, 1); if (atomic_inc_not_zero(&el->rc)) { 44 write_lock(&list_lock); rcu_read_unlock(); 45 return FAIL; 46 add_element } 47 ... ... 48 write_unlock(&list_lock); rcu_read_unlock(); 49 } } 50 3. 4. 51 release_referenced() delete() 52 { { 53 ... write_lock(&list_lock); 54 if (atomic_dec_and_test(&el->rc)) ... 55 call_rcu(&el->head, el_free); delete_element 56 ... write_unlock(&list_lock); 57 } ... 58 if (atomic_dec_and_test(&el->rc)) 59 call_rcu(&el->head, el_free); 60 ... 61 } 62 63 Sometimes, a reference to the element needs to be obtained in the 64 update (write) stream. In such cases, atomic_inc_not_zero() might be 65 overkill, since we hold the update-side spinlock. One might instead 66 use atomic_inc() in such cases.