| 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 ] |
| |
| When locking rules are violated, these 4 state bits are presented in the |
| locking error messages, inside curlies. A contrived example: |
| |
| modprobe/2287 is trying to acquire lock: |
| (&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 |
| |
| |
| The bit position indicates hardirq, softirq, hardirq-read, |
| softirq-read respectively, and the character displayed in each |
| indicates: |
| |
| '.' acquired while irqs enabled |
| '+' acquired in irq context |
| '-' acquired in process context with irqs disabled |
| '?' read-acquired both with irqs enabled and in irq context |
| |
| Unused mutexes cannot be part of the cause of an error. |
| |
| |
| 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 hierarchy 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 fashion 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 hierarchy 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. |