[PATCH v3 4/5] locking/Documentation: Add an example to help crossrelease.txt more readable

Byungchul Park
2017-11-11T13:28:03Z

Add an example explaining the rationale that the limitation that old
lockdep implies, can be relaxed.

Signed-off-by: Byungchul Park <byungchul.park@lge.com>
---
Documentation/locking/crossrelease.txt | 23 +++++++++++++++++++++++
1 file changed, 23 insertions(+)

diff --git a/Documentation/locking/crossrelease.txt b/Documentation/locking/crossrelease.txt
index bb449e8..dac56f4 100644
--- a/Documentation/locking/crossrelease.txt
+++ b/Documentation/locking/crossrelease.txt
@@ -281,6 +281,29 @@ causes a deadlock. The more lockdep adds dependencies, the more it
thoroughly works. Thus, lockdep has to do its best to detect and add as
many true dependencies to the graph as possible.

+For example:
+
+ CONTEXT X CONTEXT Y
+ --------- ---------
+ acquire A
+ acquire B /* A dependency 'A -> B' exists */
+ release B
+ release A held by Y
+
+ where A and B are different lock classes.
+
+In this case, a dependency 'A -> B' exists since:
+
+ 1. A waiter for A and a waiter for B might exist when acquiring B.
+ 2. The only way to wake up each is to release what it waits for.
+ 3. Whether the waiter for A can be woken up depends on whether the
+ other can. In other words, CONTEXT X cannot release A if it fails
+ to acquire B.
+
+Considering only typical locks, lockdep builds nothing. However,
+relaxing the limitation, a dependency 'A -> B' can be added, giving us
+more chances to check circular dependencies.
+
However, it might suffer performance degradation since
relaxing the limitation, with which design and implementation of lockdep
can be efficient, might introduce inefficiency inevitably. So lockdep
--
1.9.1