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Subject[PATCH 16/28] locking/lockdep: Add explanation to lock usage rules in lockdep design doc
Date
The irq usage and lock dependency rules that if violated a deacklock may
happen are explained in more detail.

Signed-off-by: Yuyang Du <duyuyang@gmail.com>
---
Documentation/locking/lockdep-design.txt | 33 ++++++++++++++++++++++----------
1 file changed, 23 insertions(+), 10 deletions(-)

diff --git a/Documentation/locking/lockdep-design.txt b/Documentation/locking/lockdep-design.txt
index d5062fc..e1187d8 100644
--- a/Documentation/locking/lockdep-design.txt
+++ b/Documentation/locking/lockdep-design.txt
@@ -103,14 +103,24 @@ Unused locks (e.g., mutexes) cannot be part of the cause of an error.
Single-lock state rules:
------------------------

+A lock is irq-safe means it was ever used in an irq context, while a lock
+is irq-unsafe means it was ever acquired with irq enabled.
+
A softirq-unsafe lock-class is automatically hardirq-unsafe as well. The
-following states are exclusive, and only one of them is allowed to be
-set for any lock-class:
+following states must be exclusive: only one of them is allowed to be set
+for any lock-class based on its usage:
+
+ <hardirq-safe> or <hardirq-unsafe>
+ <softirq-safe> or <softirq-unsafe>

- <hardirq-safe> and <hardirq-unsafe>
- <softirq-safe> and <softirq-unsafe>
+This is because if a lock can be used in irq context (irq-safe) then it
+cannot be ever acquired with irq enabled (irq-unsafe). Otherwise, a
+deadlock may happen. For example, in the scenario that after this lock
+was acquired but before released, if the context is interrupted this
+lock will be attempted to acquire twice, which creates a deadlock,
+referred to as lock recursion deadlock.

-The validator detects and reports lock usage that violate these
+The validator detects and reports lock usage that violates these
single-lock state rules.

Multi-lock dependency rules:
@@ -119,15 +129,18 @@ Multi-lock dependency rules:
The same lock-class must not be acquired twice, because this could lead
to lock recursion deadlocks.

-Furthermore, two locks may not be taken in different order:
+Furthermore, two locks can not be taken in inverse order:

<L1> -> <L2>
<L2> -> <L1>

-because this could lead to lock inversion deadlocks. (The validator
-finds such dependencies in arbitrary complexity, i.e. there can be any
-other locking sequence between the acquire-lock operations, the
-validator will still track all dependencies between locks.)
+because this could lead to a deadlock - referred to as lock inversion
+deadlock - as attempts to acquire the two locks form a circle which
+could lead to the two contexts waiting for each other permanently. The
+validator will find such dependency circle in arbitrary complexity,
+i.e., there can be any other locking sequence between the acquire-lock
+operations; the validator will still find whether these locks can be
+acquired in a circular fashion.

Furthermore, the following usage based lock dependencies are not allowed
between any two lock-classes:
--
1.8.3.1
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