Date:   Mon, 18 Mar 2019 16:57:16 +0800
From:   Yuyang Du <duyuyang@...il.com>
To:     peterz@...radead.org, will.deacon@....com, mingo@...nel.org
Cc:     bvanassche@....org, ming.lei@...hat.com,
        linux-kernel@...r.kernel.org, Yuyang Du <duyuyang@...il.com>
Subject: [PATCH v2 02/19] locking/lockdep: Add description and explanation in lockdep design doc

More words are added to lockdep design document regarding key concepts,
which helps people understand the design as well as read the reports.

Signed-off-by: Yuyang Du <duyuyang@...il.com>
---
 Documentation/locking/lockdep-design.txt | 89 +++++++++++++++++++++++---------
 1 file changed, 64 insertions(+), 25 deletions(-)

diff --git a/Documentation/locking/lockdep-design.txt b/Documentation/locking/lockdep-design.txt
index 49f58a0..621d8f4 100644
--- a/Documentation/locking/lockdep-design.txt
+++ b/Documentation/locking/lockdep-design.txt
@@ -10,56 +10,95 @@ Lock-class
 The basic object the validator operates upon is a 'class' of locks.
 
 A class of locks is a group of locks that are logically the same with
-respect to locking rules, even if the locks may have multiple (possibly
-tens of thousands of) instantiations. For example a lock in the inode
-struct is one class, while each inode has its own instantiation of that
-lock class.
-
-The validator tracks the 'state' of lock-classes, and it tracks
-dependencies between different lock-classes. The validator maintains a
-rolling proof that the state and the dependencies are correct.
-
-Unlike an lock instantiation, the lock-class itself never goes away: when
-a lock-class is used for the first time after bootup it gets registered,
-and all subsequent uses of that lock-class will be attached to this
-lock-class.
+respect to locking rules, even if the locks may have multiple (possibly tens
+of thousands of) instantiations. For example a lock in the inode struct is
+one class, while each inode has its own instantiation of that lock class.
+
+The validator tracks the 'usage state' of lock-classes, and it tracks the
+dependencies between different lock-classes. The dependency can be
+understood as lock order, where L1 -> L2 suggests L1 depends on L2, which
+can also be expressed as a forward dependency (L1 -> L2) or a backward
+dependency (L2 <- L1). From lockdep's perspective, the two locks (L1 and L2)
+are not necessarily related as opposed to in some modules an order must be
+followed. Here it just means that order ever happened. The validator
+maintains a continuing effort to prove that the lock usages and their
+dependencies are correct or the validator will shoot a splat if they are
+potentially incorrect.
+
+Unlike a lock instance, a lock-class itself never goes away: when a
+lock-class's instance is used for the first time after bootup the class gets
+registered, and all (subsequent) instances of that lock-class will be mapped
+to the lock-class.
 
 State
 -----
 
-The validator tracks lock-class usage history into 4 * nSTATEs + 1 separate
-state bits:
+The validator tracks lock-class usage history and divides the usage into
+(4 usages * n STATEs + 1) categories:
 
+Where the 4 usages can be:
 - 'ever held in STATE context'
 - 'ever held as readlock in STATE context'
 - 'ever held with STATE enabled'
 - 'ever held as readlock with STATE enabled'
 
-Where STATE can be either one of (kernel/locking/lockdep_states.h)
- - hardirq
- - softirq
+Where the n STATEs are coded in kernel/locking/lockdep_states.h and as of
+now they include:
+- hardirq
+- softirq
 
+Where the last 1 category is:
 - 'ever used'                                       [ == !unused        ]
 
-When locking rules are violated, these state bits are presented in the
-locking error messages, inside curlies. A contrived example:
+When locking rules are violated, these usage bits are presented in the
+locking error messages, inside curlies, with a total of 2 * n STATEs bits.
+See a contrived example:
 
    modprobe/2287 is trying to acquire lock:
-    (&sio_locks[i].lock){-.-...}, at: [<c02867fd>] mutex_lock+0x21/0x24
+    (&sio_locks[i].lock){-.-.}, at: [<c02867fd>] mutex_lock+0x21/0x24
 
    but task is already holding lock:
-    (&sio_locks[i].lock){-.-...}, at: [<c02867fd>] mutex_lock+0x21/0x24
+    (&sio_locks[i].lock){-.-.}, at: [<c02867fd>] mutex_lock+0x21/0x24
 
 
-The bit position indicates STATE, STATE-read, for each of the states listed
-above, and the character displayed in each indicates:
+For a given lock, the bit positions from left to right indicate the usage
+of the lock and readlock (if exists), for each of the n STATEs listed
+above respectively, and the character displayed at each bit position
+indicates:
 
    '.'  acquired while irqs disabled and not in irq context
    '-'  acquired in irq context
    '+'  acquired with irqs enabled
    '?'  acquired in irq context with irqs enabled.
 
-Unused mutexes cannot be part of the cause of an error.
+The bits are illustrated with an example:
+
+    (&sio_locks[i].lock){-.-.}, at: [<c02867fd>] mutex_lock+0x21/0x24
+                         ||||
+                         ||| \-> softirq disabled and not in softirq context
+                         || \--> acquired in softirq context
+                         | \---> hardirq disabled and not in hardirq context
+                          \----> acquired in hardirq context
+
+
+For a given STATE, whether the lock is ever acquired in that STATE context
+and whether that STATE is enabled yields four possible cases as shown in the
+table below. It is worth noting that the bit character is able to indicate
+which exact case is for the lock as of the reporting time.
+
+   -------------------------------------------------
+  |              | enabled in irq | disabled in irq |
+   -------------------------------------------------
+  | ever in irq  |        ?       |        -        |
+   -------------------------------------------------
+  | never in irq |        +       |        .        |
+   -------------------------------------------------
+
+The character '-' suggests irq is disabled because if otherwise, the
+charactor '?' would have been shown instead. Similar deduction can be
+applied for '+' too.
+
+Unused locks (e.g., mutexes) cannot be part of the cause of an error.
 
 
 Single-lock state rules:
-- 
1.8.3.1

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