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-rw-r--r--Documentation/RCU/rcuref.txt61
1 files changed, 59 insertions, 2 deletions
diff --git a/Documentation/RCU/rcuref.txt b/Documentation/RCU/rcuref.txt
index 4202ad0..141d531 100644
--- a/Documentation/RCU/rcuref.txt
+++ b/Documentation/RCU/rcuref.txt
@@ -20,7 +20,7 @@ release_referenced() delete()
{ {
... write_lock(&list_lock);
atomic_dec(&el->rc, relfunc) ...
- ... delete_element
+ ... remove_element
} write_unlock(&list_lock);
...
if (atomic_dec_and_test(&el->rc))
@@ -52,7 +52,7 @@ release_referenced() delete()
{ {
... spin_lock(&list_lock);
if (atomic_dec_and_test(&el->rc)) ...
- call_rcu(&el->head, el_free); delete_element
+ call_rcu(&el->head, el_free); remove_element
... spin_unlock(&list_lock);
} ...
if (atomic_dec_and_test(&el->rc))
@@ -64,3 +64,60 @@ 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.
+
+It is not always convenient to deal with "FAIL" in the
+search_and_reference() code path. In such cases, the
+atomic_dec_and_test() may be moved from delete() to el_free()
+as follows:
+
+1. 2.
+add() search_and_reference()
+{ {
+ alloc_object rcu_read_lock();
+ ... search_for_element
+ atomic_set(&el->rc, 1); atomic_inc(&el->rc);
+ spin_lock(&list_lock); ...
+
+ add_element rcu_read_unlock();
+ ... }
+ spin_unlock(&list_lock); 4.
+} delete()
+3. {
+release_referenced() spin_lock(&list_lock);
+{ ...
+ ... remove_element
+ if (atomic_dec_and_test(&el->rc)) spin_unlock(&list_lock);
+ kfree(el); ...
+ ... call_rcu(&el->head, el_free);
+} ...
+5. }
+void el_free(struct rcu_head *rhp)
+{
+ release_referenced();
+}
+
+The key point is that the initial reference added by add() is not removed
+until after a grace period has elapsed following removal. This means that
+search_and_reference() cannot find this element, which means that the value
+of el->rc cannot increase. Thus, once it reaches zero, there are no
+readers that can or ever will be able to reference the element. The
+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.
+
+In cases where delete() can sleep, synchronize_rcu() can be called from
+delete(), so that el_free() can be subsumed into delete as follows:
+
+4.
+delete()
+{
+ spin_lock(&list_lock);
+ ...
+ remove_element
+ spin_unlock(&list_lock);
+ ...
+ synchronize_rcu();
+ if (atomic_dec_and_test(&el->rc))
+ kfree(el);
+ ...
+}