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

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Date:   Thu, 28 Mar 2019 22:40:47 -0400
From:   "Joel Fernandes (Google)" <joel@...lfernandes.org>
To:     linux-kernel@...r.kernel.org
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        oleg@...hat.com, jannh@...gle.com,
        Jonathan Corbet <corbet@....net>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-doc@...r.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: [PATCH] doc/rcuref: Document real world examples in kernel

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@...hat.com
Cc: jannh@...gle.com
Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>

---
 Documentation/RCU/rcuref.txt | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/Documentation/RCU/rcuref.txt b/Documentation/RCU/rcuref.txt
index 613033ff2b9b..e5f4a49f886a 100644
--- a/Documentation/RCU/rcuref.txt
+++ b/Documentation/RCU/rcuref.txt
@@ -28,7 +28,8 @@ 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);
  					    ...
@@ -114,6 +115,11 @@ 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.
 
+The other advantage of the last pattern 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().
+
 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 +136,7 @@ delete()
     	kfree(el);
     ...
 }
+
+As additional examples in the kernel, This last pattern is also followed by
+reference counting of 'struct pid' while the prior pattern where
+atomic_inc_not_zero() is used is used by struct posix_acl.
-- 
2.21.0.392.gf8f6787159e-goog