Documentation/lockdep-design.txt

Runtime locking correctness validator
=====================================

started by Ingo Molnar <mingo@redhat.com>
additions by Arjan van de Ven <arjan@linux.intel.com>

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.

State
-----

The validator tracks lock-class usage history into 5 separate state bits:

- 'ever held in hardirq context'                    [ == hardirq-safe   ]
- 'ever held in softirq context'                    [ == softirq-safe   ]
- 'ever held with hardirqs enabled'                 [ == hardirq-unsafe ]
- 'ever held with softirqs and hardirqs enabled'    [ == softirq-unsafe ]

- 'ever used'                                       [ == !unused        ]

Single-lock state rules:
------------------------

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:

 <hardirq-safe> and <hardirq-unsafe>
 <softirq-safe> and <softirq-unsafe>

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

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:

 <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.)

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

   <hardirq-safe>   ->  <hardirq-unsafe>
   <softirq-safe>   ->  <softirq-unsafe>

The first rule comes from the fact the a hardirq-safe lock could be
taken by a hardirq context, interrupting a hardirq-unsafe lock - and
thus could result in a lock inversion deadlock. Likewise, a softirq-safe
lock could be taken by an softirq context, interrupting a softirq-unsafe
lock.

The above rules are enforced for any locking sequence that occurs in the
kernel: when acquiring a new lock, the validator checks whether there is
any rule violation between the new lock and any of the held locks.

When a lock-class changes its state, the following aspects of the above
dependency rules are enforced:

- if a new hardirq-safe lock is discovered, we check whether it
  took any hardirq-unsafe lock in the past.

- if a new softirq-safe lock is discovered, we check whether it took
  any softirq-unsafe lock in the past.

- if a new hardirq-unsafe lock is discovered, we check whether any
  hardirq-safe lock took it in the past.

- if a new softirq-unsafe lock is discovered, we check whether any
  softirq-safe lock took it in the past.

(Again, we do these checks too on the basis that an interrupt context
could interrupt _any_ of the irq-unsafe or hardirq-unsafe locks, which
could lead to a lock inversion deadlock - even if that lock scenario did
not trigger in practice yet.)

Exception: Nested data dependencies leading to nested locking
-------------------------------------------------------------

There are a few cases where the Linux kernel acquires more than one
instance of the same lock-class. Such cases typically happen when there
is some sort of hierarchy within objects of the same type. In these
cases there is an inherent "natural" ordering between the two objects
(defined by the properties of the hierarchy), and the kernel grabs the
locks in this fixed order on each of the objects.

An example of such an object hieararchy that results in "nested locking"
is that of a "whole disk" block-dev object and a "partition" block-dev
object; the partition is "part of" the whole device and as long as one
always takes the whole disk lock as a higher lock than the partition
lock, the lock ordering is fully correct. The validator does not
automatically detect this natural ordering, as the locking rule behind
the ordering is not static.

In order to teach the validator about this correct usage model, new
versions of the various locking primitives were added that allow you to
specify a "nesting level". An example call, for the block device mutex,
looks like this:

enum bdev_bd_mutex_lock_class
{
       BD_MUTEX_NORMAL,
       BD_MUTEX_WHOLE,
       BD_MUTEX_PARTITION
};

 mutex_lock_nested(&bdev->bd_contains->bd_mutex, BD_MUTEX_PARTITION);

In this case the locking is done on a bdev object that is known to be a
partition.

The validator treats a lock that is taken in such a nested fasion as a
separate (sub)class for the purposes of validation.

Note: When changing code to use the _nested() primitives, be careful and
check really thoroughly that the hiearchy is correctly mapped; otherwise
you can get false positives or false negatives.

Proof of 100% correctness:
--------------------------

The validator achieves perfect, mathematical 'closure' (proof of locking
correctness) in the sense that for every simple, standalone single-task
locking sequence that occured at least once during the lifetime of the
kernel, the validator proves it with a 100% certainty that no
combination and timing of these locking sequences can cause any class of
lock related deadlock. [*]

I.e. complex multi-CPU and multi-task locking scenarios do not have to
occur in practice to prove a deadlock: only the simple 'component'
locking chains have to occur at least once (anytime, in any
task/context) for the validator to be able to prove correctness. (For
example, complex deadlocks that would normally need more than 3 CPUs and
a very unlikely constellation of tasks, irq-contexts and timings to
occur, can be detected on a plain, lightly loaded single-CPU system as
well!)

This radically decreases the complexity of locking related QA of the
kernel: what has to be done during QA is to trigger as many "simple"
single-task locking dependencies in the kernel as possible, at least
once, to prove locking correctness - instead of having to trigger every
possible combination of locking interaction between CPUs, combined with
every possible hardirq and softirq nesting scenario (which is impossible
to do in practice).

[*] assuming that the validator itself is 100% correct, and no other
    part of the system corrupts the state of the validator in any way.
    We also assume that all NMI/SMM paths [which could interrupt
    even hardirq-disabled codepaths] are correct and do not interfere
    with the validator. We also assume that the 64-bit 'chain hash'
    value is unique for every lock-chain in the system. Also, lock
    recursion must not be higher than 20.

Performance:
------------

The above rules require _massive_ amounts of runtime checking. If we did
that for every lock taken and for every irqs-enable event, it would
render the system practically unusably slow. The complexity of checking
is O(N^2), so even with just a few hundred lock-classes we'd have to do
tens of thousands of checks for every event.

This problem is solved by checking any given 'locking scenario' (unique
sequence of locks taken after each other) only once. A simple stack of
held locks is maintained, and a lightweight 64-bit hash value is
calculated, which hash is unique for every lock chain. The hash value,
when the chain is validated for the first time, is then put into a hash
table, which hash-table can be checked in a lockfree manner. If the
locking chain occurs again later on, the hash table tells us that we
dont have to validate the chain again.