1 Runtime locking correctness validator
2 =====================================
4 started by Ingo Molnar <mingo@redhat.com>
5 additions by Arjan van de Ven <arjan@linux.intel.com>
10 The basic object the validator operates upon is a 'class' of locks.
12 A class of locks is a group of locks that are logically the same with
13 respect to locking rules, even if the locks may have multiple (possibly
14 tens of thousands of) instantiations. For example a lock in the inode
15 struct is one class, while each inode has its own instantiation of that
18 The validator tracks the 'state' of lock-classes, and it tracks
19 dependencies between different lock-classes. The validator maintains a
20 rolling proof that the state and the dependencies are correct.
22 Unlike an lock instantiation, the lock-class itself never goes away: when
23 a lock-class is used for the first time after bootup it gets registered,
24 and all subsequent uses of that lock-class will be attached to this
30 The validator tracks lock-class usage history into 5 separate state bits:
32 - 'ever held in hardirq context' [ == hardirq-safe ]
33 - 'ever held in softirq context' [ == softirq-safe ]
34 - 'ever held with hardirqs enabled' [ == hardirq-unsafe ]
35 - 'ever held with softirqs and hardirqs enabled' [ == softirq-unsafe ]
37 - 'ever used' [ == !unused ]
39 When locking rules are violated, these 4 state bits are presented in the
40 locking error messages, inside curlies. A contrived example:
42 modprobe/2287 is trying to acquire lock:
43 (&sio_locks[i].lock){--..}, at: [<c02867fd>] mutex_lock+0x21/0x24
45 but task is already holding lock:
46 (&sio_locks[i].lock){--..}, at: [<c02867fd>] mutex_lock+0x21/0x24
49 The bit position indicates hardirq, softirq, hardirq-read,
50 softirq-read respectively, and the character displayed in each
53 '.' acquired while irqs disabled
54 '+' acquired in irq context
55 '-' acquired with irqs enabled
56 '?' read acquired in irq context with irqs enabled.
58 Unused mutexes cannot be part of the cause of an error.
61 Single-lock state rules:
62 ------------------------
64 A softirq-unsafe lock-class is automatically hardirq-unsafe as well. The
65 following states are exclusive, and only one of them is allowed to be
66 set for any lock-class:
68 <hardirq-safe> and <hardirq-unsafe>
69 <softirq-safe> and <softirq-unsafe>
71 The validator detects and reports lock usage that violate these
72 single-lock state rules.
74 Multi-lock dependency rules:
75 ----------------------------
77 The same lock-class must not be acquired twice, because this could lead
78 to lock recursion deadlocks.
80 Furthermore, two locks may not be taken in different order:
85 because this could lead to lock inversion deadlocks. (The validator
86 finds such dependencies in arbitrary complexity, i.e. there can be any
87 other locking sequence between the acquire-lock operations, the
88 validator will still track all dependencies between locks.)
90 Furthermore, the following usage based lock dependencies are not allowed
91 between any two lock-classes:
93 <hardirq-safe> -> <hardirq-unsafe>
94 <softirq-safe> -> <softirq-unsafe>
96 The first rule comes from the fact the a hardirq-safe lock could be
97 taken by a hardirq context, interrupting a hardirq-unsafe lock - and
98 thus could result in a lock inversion deadlock. Likewise, a softirq-safe
99 lock could be taken by an softirq context, interrupting a softirq-unsafe
102 The above rules are enforced for any locking sequence that occurs in the
103 kernel: when acquiring a new lock, the validator checks whether there is
104 any rule violation between the new lock and any of the held locks.
106 When a lock-class changes its state, the following aspects of the above
107 dependency rules are enforced:
109 - if a new hardirq-safe lock is discovered, we check whether it
110 took any hardirq-unsafe lock in the past.
112 - if a new softirq-safe lock is discovered, we check whether it took
113 any softirq-unsafe lock in the past.
115 - if a new hardirq-unsafe lock is discovered, we check whether any
116 hardirq-safe lock took it in the past.
118 - if a new softirq-unsafe lock is discovered, we check whether any
119 softirq-safe lock took it in the past.
121 (Again, we do these checks too on the basis that an interrupt context
122 could interrupt _any_ of the irq-unsafe or hardirq-unsafe locks, which
123 could lead to a lock inversion deadlock - even if that lock scenario did
124 not trigger in practice yet.)
126 Exception: Nested data dependencies leading to nested locking
127 -------------------------------------------------------------
129 There are a few cases where the Linux kernel acquires more than one
130 instance of the same lock-class. Such cases typically happen when there
131 is some sort of hierarchy within objects of the same type. In these
132 cases there is an inherent "natural" ordering between the two objects
133 (defined by the properties of the hierarchy), and the kernel grabs the
134 locks in this fixed order on each of the objects.
136 An example of such an object hierarchy that results in "nested locking"
137 is that of a "whole disk" block-dev object and a "partition" block-dev
138 object; the partition is "part of" the whole device and as long as one
139 always takes the whole disk lock as a higher lock than the partition
140 lock, the lock ordering is fully correct. The validator does not
141 automatically detect this natural ordering, as the locking rule behind
142 the ordering is not static.
144 In order to teach the validator about this correct usage model, new
145 versions of the various locking primitives were added that allow you to
146 specify a "nesting level". An example call, for the block device mutex,
149 enum bdev_bd_mutex_lock_class
156 mutex_lock_nested(&bdev->bd_contains->bd_mutex, BD_MUTEX_PARTITION);
158 In this case the locking is done on a bdev object that is known to be a
161 The validator treats a lock that is taken in such a nested fashion as a
162 separate (sub)class for the purposes of validation.
164 Note: When changing code to use the _nested() primitives, be careful and
165 check really thoroughly that the hierarchy is correctly mapped; otherwise
166 you can get false positives or false negatives.
168 Proof of 100% correctness:
169 --------------------------
171 The validator achieves perfect, mathematical 'closure' (proof of locking
172 correctness) in the sense that for every simple, standalone single-task
173 locking sequence that occurred at least once during the lifetime of the
174 kernel, the validator proves it with a 100% certainty that no
175 combination and timing of these locking sequences can cause any class of
176 lock related deadlock. [*]
178 I.e. complex multi-CPU and multi-task locking scenarios do not have to
179 occur in practice to prove a deadlock: only the simple 'component'
180 locking chains have to occur at least once (anytime, in any
181 task/context) for the validator to be able to prove correctness. (For
182 example, complex deadlocks that would normally need more than 3 CPUs and
183 a very unlikely constellation of tasks, irq-contexts and timings to
184 occur, can be detected on a plain, lightly loaded single-CPU system as
187 This radically decreases the complexity of locking related QA of the
188 kernel: what has to be done during QA is to trigger as many "simple"
189 single-task locking dependencies in the kernel as possible, at least
190 once, to prove locking correctness - instead of having to trigger every
191 possible combination of locking interaction between CPUs, combined with
192 every possible hardirq and softirq nesting scenario (which is impossible
195 [*] assuming that the validator itself is 100% correct, and no other
196 part of the system corrupts the state of the validator in any way.
197 We also assume that all NMI/SMM paths [which could interrupt
198 even hardirq-disabled codepaths] are correct and do not interfere
199 with the validator. We also assume that the 64-bit 'chain hash'
200 value is unique for every lock-chain in the system. Also, lock
201 recursion must not be higher than 20.
206 The above rules require _massive_ amounts of runtime checking. If we did
207 that for every lock taken and for every irqs-enable event, it would
208 render the system practically unusably slow. The complexity of checking
209 is O(N^2), so even with just a few hundred lock-classes we'd have to do
210 tens of thousands of checks for every event.
212 This problem is solved by checking any given 'locking scenario' (unique
213 sequence of locks taken after each other) only once. A simple stack of
214 held locks is maintained, and a lightweight 64-bit hash value is
215 calculated, which hash is unique for every lock chain. The hash value,
216 when the chain is validated for the first time, is then put into a hash
217 table, which hash-table can be checked in a lockfree manner. If the
218 locking chain occurs again later on, the hash table tells us that we
219 dont have to validate the chain again.