diff options
| -rw-r--r-- | Documentation/RCU/rcuref.txt | 21 |
1 files changed, 20 insertions, 1 deletions
diff --git a/Documentation/RCU/rcuref.txt b/Documentation/RCU/rcuref.txt index 613033ff2b9b..5e6429d66c24 100644 --- a/Documentation/RCU/rcuref.txt +++ b/Documentation/RCU/rcuref.txt @@ -12,6 +12,7 @@ please read on. Reference counting on elements of lists which are protected by traditional reader/writer spinlocks or semaphores are straightforward: +CODE LISTING A: 1. 2. add() search_and_reference() { { @@ -28,7 +29,8 @@ add() search_and_reference() release_referenced() delete() { { ... write_lock(&list_lock); - atomic_dec(&el->rc, relfunc) ... + if(atomic_dec_and_test(&el->rc)) ... + kfree(el); ... remove_element } write_unlock(&list_lock); ... @@ -44,6 +46,7 @@ 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: +CODE LISTING B: 1. 2. add() search_and_reference() { { @@ -79,6 +82,7 @@ search_and_reference() code path. In such cases, the atomic_dec_and_test() may be moved from delete() to el_free() as follows: +CODE LISTING C: 1. 2. add() search_and_reference() { { @@ -114,6 +118,17 @@ element can therefore safely be freed. This in turn guarantees that if any reader finds the element, that reader may safely acquire a reference without checking the value of the reference counter. +A clear advantage of the RCU-based pattern in listing C over the one +in listing B is that any call to search_and_reference() that locates +a given object will succeed in obtaining a reference to that object, +even given a concurrent invocation of delete() for that same object. +Similarly, a clear advantage of both listings B and C over listing A is +that a call to delete() is not delayed even if there are an arbitrarily +large number of calls to search_and_reference() searching for the same +object that delete() was invoked on. Instead, all that is delayed is +the eventual invocation of kfree(), which is usually not a problem on +modern computer systems, even the small ones. + In cases where delete() can sleep, synchronize_rcu() can be called from delete(), so that el_free() can be subsumed into delete as follows: @@ -130,3 +145,7 @@ delete() kfree(el); ... } + +As additional examples in the kernel, the pattern in listing C is used by +reference counting of struct pid, while the pattern in listing B is used by +struct posix_acl. |