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

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Date:   Sat, 11 Nov 2017 22:26:31 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     peterz@...radead.org, mingo@...nel.org
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-block@...r.kernel.org,
        kernel-team@....com
Subject: [PATCH v3 4/5] locking/Documentation: Add an example to help crossrelease.txt more readable

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

Signed-off-by: Byungchul Park <byungchul.park@....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