diff options
| author | Paul E. McKenney <paulmck@us.ibm.com> | 2006-02-01 03:06:42 -0800 |
|---|---|---|
| committer | Linus Torvalds <torvalds@g5.osdl.org> | 2006-02-01 08:53:25 -0800 |
| commit | d19720a909b4443f78cbb03f4f090180e143ad9d (patch) | |
| tree | 56e579612d82f4b30d5cb943df1079b0b5f4700a /Documentation/RCU/rcuref.txt | |
| parent | 53d8be5c144ece5d48745810b14248968e73eaf2 (diff) | |
| download | lwn-d19720a909b4443f78cbb03f4f090180e143ad9d.tar.gz lwn-d19720a909b4443f78cbb03f4f090180e143ad9d.zip | |
[PATCH] RCU documentation fixes (January 2006 update)
Updates to in-tree RCU documentation based on comments over the past few
months.
Signed-off-by: "Paul E. McKenney" <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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 3f60db41b2f0..451de2ad8329 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. |