1Runtime locking correctness validator 2===================================== 3 4started by Ingo Molnar <mingo@redhat.com> 5additions by Arjan van de Ven <arjan@linux.intel.com> 6 7Lock-class 8---------- 9 10The basic object the validator operates upon is a 'class' of locks. 11 12A class of locks is a group of locks that are logically the same with 13respect to locking rules, even if the locks may have multiple (possibly 14tens of thousands of) instantiations. For example a lock in the inode 15struct is one class, while each inode has its own instantiation of that 16lock class. 17 18The validator tracks the 'state' of lock-classes, and it tracks 19dependencies between different lock-classes. The validator maintains a 20rolling proof that the state and the dependencies are correct. 21 22Unlike an lock instantiation, the lock-class itself never goes away: when 23a lock-class is used for the first time after bootup it gets registered, 24and all subsequent uses of that lock-class will be attached to this 25lock-class. 26 27State 28----- 29 30The validator tracks lock-class usage history into 4n + 1 separate state bits: 31 32- 'ever held in STATE context' 33- 'ever head as readlock in STATE context' 34- 'ever head with STATE enabled' 35- 'ever head as readlock with STATE enabled' 36 37Where STATE can be either one of (kernel/lockdep_states.h) 38 - hardirq 39 - softirq 40 - reclaim_fs 41 42- 'ever used' [ == !unused ] 43 44When locking rules are violated, these state bits are presented in the 45locking error messages, inside curlies. A contrived example: 46 47 modprobe/2287 is trying to acquire lock: 48 (&sio_locks[i].lock){-.-...}, at: [<c02867fd>] mutex_lock+0x21/0x24 49 50 but task is already holding lock: 51 (&sio_locks[i].lock){-.-...}, at: [<c02867fd>] mutex_lock+0x21/0x24 52 53 54The bit position indicates STATE, STATE-read, for each of the states listed 55above, and the character displayed in each indicates: 56 57 '.' acquired while irqs disabled and not in irq context 58 '-' acquired in irq context 59 '+' acquired with irqs enabled 60 '?' acquired in irq context with irqs enabled. 61 62Unused mutexes cannot be part of the cause of an error. 63 64 65Single-lock state rules: 66------------------------ 67 68A softirq-unsafe lock-class is automatically hardirq-unsafe as well. The 69following states are exclusive, and only one of them is allowed to be 70set for any lock-class: 71 72 <hardirq-safe> and <hardirq-unsafe> 73 <softirq-safe> and <softirq-unsafe> 74 75The validator detects and reports lock usage that violate these 76single-lock state rules. 77 78Multi-lock dependency rules: 79---------------------------- 80 81The same lock-class must not be acquired twice, because this could lead 82to lock recursion deadlocks. 83 84Furthermore, two locks may not be taken in different order: 85 86 <L1> -> <L2> 87 <L2> -> <L1> 88 89because this could lead to lock inversion deadlocks. (The validator 90finds such dependencies in arbitrary complexity, i.e. there can be any 91other locking sequence between the acquire-lock operations, the 92validator will still track all dependencies between locks.) 93 94Furthermore, the following usage based lock dependencies are not allowed 95between any two lock-classes: 96 97 <hardirq-safe> -> <hardirq-unsafe> 98 <softirq-safe> -> <softirq-unsafe> 99 100The first rule comes from the fact the a hardirq-safe lock could be 101taken by a hardirq context, interrupting a hardirq-unsafe lock - and 102thus could result in a lock inversion deadlock. Likewise, a softirq-safe 103lock could be taken by an softirq context, interrupting a softirq-unsafe 104lock. 105 106The above rules are enforced for any locking sequence that occurs in the 107kernel: when acquiring a new lock, the validator checks whether there is 108any rule violation between the new lock and any of the held locks. 109 110When a lock-class changes its state, the following aspects of the above 111dependency rules are enforced: 112 113- if a new hardirq-safe lock is discovered, we check whether it 114 took any hardirq-unsafe lock in the past. 115 116- if a new softirq-safe lock is discovered, we check whether it took 117 any softirq-unsafe lock in the past. 118 119- if a new hardirq-unsafe lock is discovered, we check whether any 120 hardirq-safe lock took it in the past. 121 122- if a new softirq-unsafe lock is discovered, we check whether any 123 softirq-safe lock took it in the past. 124 125(Again, we do these checks too on the basis that an interrupt context 126could interrupt _any_ of the irq-unsafe or hardirq-unsafe locks, which 127could lead to a lock inversion deadlock - even if that lock scenario did 128not trigger in practice yet.) 129 130Exception: Nested data dependencies leading to nested locking 131------------------------------------------------------------- 132 133There are a few cases where the Linux kernel acquires more than one 134instance of the same lock-class. Such cases typically happen when there 135is some sort of hierarchy within objects of the same type. In these 136cases there is an inherent "natural" ordering between the two objects 137(defined by the properties of the hierarchy), and the kernel grabs the 138locks in this fixed order on each of the objects. 139 140An example of such an object hierarchy that results in "nested locking" 141is that of a "whole disk" block-dev object and a "partition" block-dev 142object; the partition is "part of" the whole device and as long as one 143always takes the whole disk lock as a higher lock than the partition 144lock, the lock ordering is fully correct. The validator does not 145automatically detect this natural ordering, as the locking rule behind 146the ordering is not static. 147 148In order to teach the validator about this correct usage model, new 149versions of the various locking primitives were added that allow you to 150specify a "nesting level". An example call, for the block device mutex, 151looks like this: 152 153enum bdev_bd_mutex_lock_class 154{ 155 BD_MUTEX_NORMAL, 156 BD_MUTEX_WHOLE, 157 BD_MUTEX_PARTITION 158}; 159 160 mutex_lock_nested(&bdev->bd_contains->bd_mutex, BD_MUTEX_PARTITION); 161 162In this case the locking is done on a bdev object that is known to be a 163partition. 164 165The validator treats a lock that is taken in such a nested fashion as a 166separate (sub)class for the purposes of validation. 167 168Note: When changing code to use the _nested() primitives, be careful and 169check really thoroughly that the hierarchy is correctly mapped; otherwise 170you can get false positives or false negatives. 171 172Proof of 100% correctness: 173-------------------------- 174 175The validator achieves perfect, mathematical 'closure' (proof of locking 176correctness) in the sense that for every simple, standalone single-task 177locking sequence that occurred at least once during the lifetime of the 178kernel, the validator proves it with a 100% certainty that no 179combination and timing of these locking sequences can cause any class of 180lock related deadlock. [*] 181 182I.e. complex multi-CPU and multi-task locking scenarios do not have to 183occur in practice to prove a deadlock: only the simple 'component' 184locking chains have to occur at least once (anytime, in any 185task/context) for the validator to be able to prove correctness. (For 186example, complex deadlocks that would normally need more than 3 CPUs and 187a very unlikely constellation of tasks, irq-contexts and timings to 188occur, can be detected on a plain, lightly loaded single-CPU system as 189well!) 190 191This radically decreases the complexity of locking related QA of the 192kernel: what has to be done during QA is to trigger as many "simple" 193single-task locking dependencies in the kernel as possible, at least 194once, to prove locking correctness - instead of having to trigger every 195possible combination of locking interaction between CPUs, combined with 196every possible hardirq and softirq nesting scenario (which is impossible 197to do in practice). 198 199[*] assuming that the validator itself is 100% correct, and no other 200 part of the system corrupts the state of the validator in any way. 201 We also assume that all NMI/SMM paths [which could interrupt 202 even hardirq-disabled codepaths] are correct and do not interfere 203 with the validator. We also assume that the 64-bit 'chain hash' 204 value is unique for every lock-chain in the system. Also, lock 205 recursion must not be higher than 20. 206 207Performance: 208------------ 209 210The above rules require _massive_ amounts of runtime checking. If we did 211that for every lock taken and for every irqs-enable event, it would 212render the system practically unusably slow. The complexity of checking 213is O(N^2), so even with just a few hundred lock-classes we'd have to do 214tens of thousands of checks for every event. 215 216This problem is solved by checking any given 'locking scenario' (unique 217sequence of locks taken after each other) only once. A simple stack of 218held locks is maintained, and a lightweight 64-bit hash value is 219calculated, which hash is unique for every lock chain. The hash value, 220when the chain is validated for the first time, is then put into a hash 221table, which hash-table can be checked in a lockfree manner. If the 222locking chain occurs again later on, the hash table tells us that we 223dont have to validate the chain again. 224