[PATCH v2] doc/rcuref: Document real world examples in kernel

[Joel Fernandes (Google)]

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