diff options
| author | Dave Kleikamp <shaggy@austin.ibm.com> | 2006-03-14 17:05:45 -0600 |
|---|---|---|
| committer | Dave Kleikamp <shaggy@austin.ibm.com> | 2006-03-14 17:05:45 -0600 |
| commit | c5111f504d2a9b0d258d7c4752b4093523315989 (patch) | |
| tree | 6a52864aff79691689aea21cb0cb928327d5de5b /Documentation/RCU/rcuref.txt | |
| parent | 69eb66d7da7dba2696281981347698e1693c2340 (diff) | |
| parent | a488edc914aa1d766a4e2c982b5ae03d5657ec1b (diff) | |
Merge with /home/shaggy/git/linus-clean/
Diffstat (limited to 'Documentation/RCU/rcuref.txt')
| -rw-r--r-- | Documentation/RCU/rcuref.txt | 31 |
1 files changed, 15 insertions, 16 deletions
diff --git a/Documentation/RCU/rcuref.txt b/Documentation/RCU/rcuref.txt index 3f60db41b2f..451de2ad832 100644 --- a/Documentation/RCU/rcuref.txt +++ b/Documentation/RCU/rcuref.txt @@ -1,7 +1,7 @@ -Refcounter design for elements of lists/arrays protected by RCU. +Reference-count design 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: +Reference counting on elements of lists which are protected by traditional +reader/writer spinlocks or semaphores are straightforward: 1. 2. add() search_and_reference() @@ -28,12 +28,12 @@ release_referenced() delete() ... } -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 +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_get in search_and_reference could potentially hold reference to an element which -has already been deleted from the list/array. atomic_inc_not_zero takes -care of this scenario. search_and_reference should look as; +has already been deleted from the list/array. Use atomic_inc_not_zero() +in this scenario as follows: 1. 2. add() search_and_reference() @@ -51,17 +51,16 @@ add() search_and_reference() 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)) ... + call_rcu(&el->head, el_free); delete_element + ... write_unlock(&list_lock); +} ... if (atomic_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, atomic_inc_not_zero might be an -overkill since the spinlock serialising list updates are held. atomic_inc -is to be used in such cases. - +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. |