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Diffstat (limited to 'Documentation/RCU/rcuref.txt')
| -rw-r--r-- | Documentation/RCU/rcuref.txt | 66 |
1 files changed, 66 insertions, 0 deletions
diff --git a/Documentation/RCU/rcuref.txt b/Documentation/RCU/rcuref.txt new file mode 100644 index 00000000..4202ad09 --- /dev/null +++ b/Documentation/RCU/rcuref.txt @@ -0,0 +1,66 @@ +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. |