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Message-Id: <20190329024047.206989-1-joel@joelfernandes.org>
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
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