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 5 separate state bits: 31 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 ] 36 37- 'ever used' [ == !unused ] 38 39When locking rules are violated, these 4 state bits are presented in the 40locking error messages, inside curlies. A contrived example: 41 42 modprobe/2287 is trying to acquire lock: 43 (&sio_locks[i].lock){--..}, at: [<c02867fd>] mutex_lock+0x21/0x24 44 45 but task is already holding lock: 46 (&sio_locks[i].lock){--..}, at: [<c02867fd>] mutex_lock+0x21/0x24 47 48 49The bit position indicates hardirq, softirq, hardirq-read, 50softirq-read respectively, and the character displayed in each 51indicates: 52 53 '.' acquired while irqs disabled 54 '+' acquired in irq context 55 '-' acquired with irqs enabled 56 '?' read acquired in irq context with irqs enabled. 57 58Unused mutexes cannot be part of the cause of an error. 59 60 61Single-lock state rules: 62------------------------ 63 64A softirq-unsafe lock-class is automatically hardirq-unsafe as well. The 65following states are exclusive, and only one of them is allowed to be 66set for any lock-class: 67 68 <hardirq-safe> and <hardirq-unsafe> 69 <softirq-safe> and <softirq-unsafe> 70 71The validator detects and reports lock usage that violate these 72single-lock state rules. 73 74Multi-lock dependency rules: 75---------------------------- 76 77The same lock-class must not be acquired twice, because this could lead 78to lock recursion deadlocks. 79 80Furthermore, two locks may not be taken in different order: 81 82 <L1> -> <L2> 83 <L2> -> <L1> 84 85because this could lead to lock inversion deadlocks. (The validator 86finds such dependencies in arbitrary complexity, i.e. there can be any 87other locking sequence between the acquire-lock operations, the 88validator will still track all dependencies between locks.) 89 90Furthermore, the following usage based lock dependencies are not allowed 91between any two lock-classes: 92 93 <hardirq-safe> -> <hardirq-unsafe> 94 <softirq-safe> -> <softirq-unsafe> 95 96The first rule comes from the fact the a hardirq-safe lock could be 97taken by a hardirq context, interrupting a hardirq-unsafe lock - and 98thus could result in a lock inversion deadlock. Likewise, a softirq-safe 99lock could be taken by an softirq context, interrupting a softirq-unsafe 100lock. 101 102The above rules are enforced for any locking sequence that occurs in the 103kernel: when acquiring a new lock, the validator checks whether there is 104any rule violation between the new lock and any of the held locks. 105 106When a lock-class changes its state, the following aspects of the above 107dependency rules are enforced: 108 109- if a new hardirq-safe lock is discovered, we check whether it 110 took any hardirq-unsafe lock in the past. 111 112- if a new softirq-safe lock is discovered, we check whether it took 113 any softirq-unsafe lock in the past. 114 115- if a new hardirq-unsafe lock is discovered, we check whether any 116 hardirq-safe lock took it in the past. 117 118- if a new softirq-unsafe lock is discovered, we check whether any 119 softirq-safe lock took it in the past. 120 121(Again, we do these checks too on the basis that an interrupt context 122could interrupt _any_ of the irq-unsafe or hardirq-unsafe locks, which 123could lead to a lock inversion deadlock - even if that lock scenario did 124not trigger in practice yet.) 125 126Exception: Nested data dependencies leading to nested locking 127------------------------------------------------------------- 128 129There are a few cases where the Linux kernel acquires more than one 130instance of the same lock-class. Such cases typically happen when there 131is some sort of hierarchy within objects of the same type. In these 132cases there is an inherent "natural" ordering between the two objects 133(defined by the properties of the hierarchy), and the kernel grabs the 134locks in this fixed order on each of the objects. 135 136An example of such an object hierarchy that results in "nested locking" 137is that of a "whole disk" block-dev object and a "partition" block-dev 138object; the partition is "part of" the whole device and as long as one 139always takes the whole disk lock as a higher lock than the partition 140lock, the lock ordering is fully correct. The validator does not 141automatically detect this natural ordering, as the locking rule behind 142the ordering is not static. 143 144In order to teach the validator about this correct usage model, new 145versions of the various locking primitives were added that allow you to 146specify a "nesting level". An example call, for the block device mutex, 147looks like this: 148 149enum bdev_bd_mutex_lock_class 150{ 151 BD_MUTEX_NORMAL, 152 BD_MUTEX_WHOLE, 153 BD_MUTEX_PARTITION 154}; 155 156 mutex_lock_nested(&bdev->bd_contains->bd_mutex, BD_MUTEX_PARTITION); 157 158In this case the locking is done on a bdev object that is known to be a 159partition. 160 161The validator treats a lock that is taken in such a nested fashion as a 162separate (sub)class for the purposes of validation. 163 164Note: When changing code to use the _nested() primitives, be careful and 165check really thoroughly that the hierarchy is correctly mapped; otherwise 166you can get false positives or false negatives. 167 168Proof of 100% correctness: 169-------------------------- 170 171The validator achieves perfect, mathematical 'closure' (proof of locking 172correctness) in the sense that for every simple, standalone single-task 173locking sequence that occurred at least once during the lifetime of the 174kernel, the validator proves it with a 100% certainty that no 175combination and timing of these locking sequences can cause any class of 176lock related deadlock. [*] 177 178I.e. complex multi-CPU and multi-task locking scenarios do not have to 179occur in practice to prove a deadlock: only the simple 'component' 180locking chains have to occur at least once (anytime, in any 181task/context) for the validator to be able to prove correctness. (For 182example, complex deadlocks that would normally need more than 3 CPUs and 183a very unlikely constellation of tasks, irq-contexts and timings to 184occur, can be detected on a plain, lightly loaded single-CPU system as 185well!) 186 187This radically decreases the complexity of locking related QA of the 188kernel: what has to be done during QA is to trigger as many "simple" 189single-task locking dependencies in the kernel as possible, at least 190once, to prove locking correctness - instead of having to trigger every 191possible combination of locking interaction between CPUs, combined with 192every possible hardirq and softirq nesting scenario (which is impossible 193to do in practice). 194 195[*] 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. 202 203Performance: 204------------ 205 206The above rules require _massive_ amounts of runtime checking. If we did 207that for every lock taken and for every irqs-enable event, it would 208render the system practically unusably slow. The complexity of checking 209is O(N^2), so even with just a few hundred lock-classes we'd have to do 210tens of thousands of checks for every event. 211 212This problem is solved by checking any given 'locking scenario' (unique 213sequence of locks taken after each other) only once. A simple stack of 214held locks is maintained, and a lightweight 64-bit hash value is 215calculated, which hash is unique for every lock chain. The hash value, 216when the chain is validated for the first time, is then put into a hash 217table, which hash-table can be checked in a lockfree manner. If the 218locking chain occurs again later on, the hash table tells us that we 219dont have to validate the chain again. 220