Document similar real world examples in the kernel corresponding to the
second and third code snippets. Also correct an issue in
release_referenced() in the code snippet example.
Cc: oleg@xxxxxxxxxx
Cc: jannh@xxxxxxxxxx
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
v1->v2:
- minor fixups, label code listings.
Documentation/RCU/rcuref.txt | 17 ++++++++++++++++-
1 file changed, 16 insertions(+), 1 deletion(-)
diff --git a/Documentation/RCU/rcuref.txt b/Documentation/RCU/rcuref.txt
index 613033ff2b9b..a49d525ce975 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 +118,13 @@ 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.
+As can be seen, a clear advantage of the pattern in listing C is, if there are
+several calls to search_and_reference() in parallel to the delete(), then all
+of those will succeed in obtaining a reference to the object if the object
+could be found in the list before it was deleted in delete(), unlike the
+pattern in listing B which would fail to acquire references in such a situation
+even though the object is still in memory.
+
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 +141,7 @@ 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.
--
2.21.0.392.gf8f6787159e-goog
