Choose the detector from the failure you are trying to prove: use KASAN for invalid memory accesses such as out-of-bounds reads and use-after-free, KCSAN for unsynchronized concurrent accesses and data races, and lockdep for lock-ordering defects and potential deadlocks. They are complementary rather than interchangeable. When the symptom is ambiguous, combine the smallest set that covers the plausible causes, then exercise the instrumented kernel with repeatable KUnit or kselftest workloads.
The short answer
The three tools observe different correctness properties:
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| Detector | Primary signal | How it works | Best first choice | Main constraint |
|---|---|---|---|---|
| KCSAN | Concurrent data races | Compile-time instrumentation plus watchpoint-based sampling | A suspected unsynchronized shared-memory access | Sampling can miss races; tuning changes both observation probability and runtime cost |
| KASAN | Out-of-bounds and use-after-free memory errors | Compile-time checks in software modes or arm64 memory tagging in hardware mode | An invalid pointer, slab, stack, page, or vmalloc access | Generic mode has substantial memory and performance overhead; tag-based modes are arm64-only in the cited v6.6 documentation |
| lockdep | Lock-ordering errors and potential deadlocks | Runtime tracking of lock acquisitions and releases, analyzed as a dependency graph | ABBA ordering, recursive locking, missing lock coverage, or a deadlock warning | It validates locking relationships, not arbitrary memory corruption or every data race |
A KASAN report does not prove that a lock is wrong, and a lockdep warning does not prove that a pointer is corrupt. A race may produce a later KASAN failure, while a deadlock may leave no invalid memory access at all. Start with the earliest trustworthy symptom and instrument for that class.
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KASAN: an invalid memory access
Kernel Address Sanitizer is a dynamic memory-safety detector designed to find out-of-bounds and use-after-free bugs. Its checks can catch accesses outside an allocated object, accesses through memory whose lifetime has ended, and related violations involving kernel memory regions. The report normally identifies the failing access, the accessed address, the allocation or free stack, and a shadow-memory diagnosis.
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That evidence answers a lifetime or bounds question: “Was this address valid for this access at this point?” It does not by itself establish why two CPUs reached the access concurrently or whether a missing lock allowed the lifetime violation. If the trace also suggests concurrency, use KCSAN or lockdep as a second investigation rather than treating KASAN as a race detector.
KCSAN: a concurrent data race
The Kernel Concurrency Sanitizer is a dynamic race detector. It relies on compile-time instrumentation and a watchpoint-based sampling approach to detect races. Instrumented accesses are observed against watchpoints held for a configurable delay; conflicting accesses that are not ordered by the kernel’s synchronization primitives can trigger a report.
Because KCSAN samples rather than checking every access continuously, a clean run is not proof that no race exists. The chance of observing a race depends on the workload, timing, watchpoint frequency, and delay settings. Repeating a test, changing those runtime controls, or increasing concurrency can expose a race that one run misses.
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lockdep: a locking relationship that can fail
lockdep is a locking correctness validator. It records lock acquisition and release relationships at runtime and builds a dependency graph. When a new edge would create an invalid cycle, or when locking rules such as recursive acquisition are violated, lockdep reports the path and the involved lock classes.
Its strongest use case is a potential deadlock: thread A takes lock X then Y, while another path takes Y then X. It can also expose incorrect recursive locking and gaps in the locking model. Lockdep does not inspect every ordinary load and store, so it cannot replace KCSAN for a data race or KASAN for a bad pointer.
KASAN in detail
Choose the KASAN mode that matches the target
The Linux documentation describes three KASAN modes selected through kernel configuration:
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- Generic KASAN uses software instrumentation and supports many architectures. It is intended primarily for debugging and carries significant memory and performance overhead.
- Software tag-based KASAN uses software memory tags on arm64. It is intended for lower-overhead arm64 testing compared with the generic implementation.
- Hardware tag-based KASAN relies on arm64 Memory Tagging Extension (MTE). It is intended for in-field detection or security mitigation rather than the broadest debugging coverage.
The cited v6.6 documentation lists the two tag-based modes as arm64-only. Do not select a tag-based mode for a non-arm64 target, and verify that the processor and kernel configuration support MTE before expecting hardware tagging to work.
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Build and enable KASAN
Start with CONFIG_KASAN=y, then select exactly the implementation appropriate for the build: CONFIG_KASAN_GENERIC, CONFIG_KASAN_SW_TAGS, or CONFIG_KASAN_HW_TAGS. The generic option is the usual first choice when architecture coverage matters; software or hardware tags are options for supported arm64 systems where their different overhead and deployment goals fit the test.
- Configure the kernel with
CONFIG_KASAN=yand one KASAN mode. - Build and boot that kernel on a test system; keep a non-KASAN kernel available for recovery because instrumented kernels consume more memory and run more slowly.
- Run the smallest reproducer first, then execute the broader KUnit or kselftest workload that exercises the affected subsystem.
- Save the complete report, including the access stack and allocation/free stacks, before changing code. Those stacks are needed to distinguish a direct bug from corruption detected later.
- Fix the bounds or lifetime defect, rebuild, and rerun the same workload to confirm that the report disappears without introducing a different failure.
When KASAN is the right first move
- A faulting address lies just beyond a slab object or array.
- The kernel reports a use-after-free, double access to freed memory, or an invalid stack, page, or vmalloc address.
- The crash is deterministic enough that a memory checker can reach the failing path.
- You need allocation and free history to identify which subsystem ended an object’s lifetime too early.
If the suspected defect is a race that occasionally corrupts a pointer, KASAN may catch the consequence but not the cause. Pair it with KCSAN when unsynchronized ownership or lifetime transitions are plausible.
KCSAN in detail
What sampling means in practice
KCSAN instruments memory accesses at compile time and places watchpoints on sampled accesses. A conflicting access observed while a watchpoint is active can produce a race report. The detector therefore trades exhaustive checking for a practical runtime cost. Watchpoint frequency and delay parameters affect both the probability of observing a short-lived race and the amount of instrumentation work.
Use repeated runs and varied schedules as part of the method. A single pass that produces no report is only evidence about that particular execution. Increase stress, CPU parallelism, interrupt activity, and test repetition before concluding that a race is absent.
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Build and configure KCSAN
Enable CONFIG_KCSAN=y. The cited KCSAN documentation requires GCC 11 or newer, or Clang 11 or newer, for the compiler instrumentation. Runtime controls are exposed through debugfs and module parameters, allowing a test to adjust sampling behavior without rebuilding every time.
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- Build a kernel with
CONFIG_KCSAN=yand a supported compiler (GCC 11+ or Clang 11+). - Boot it with debugfs available, then inspect the KCSAN controls exposed there and the applicable module parameters.
- Run the race reproducer repeatedly under load. Record the control values with each run so that a “no report” result remains interpretable.
- If the race is rare, increase the number of iterations or adjust watchpoint frequency and delay. Treat the resulting overhead as a test-setting choice, not a production performance estimate.
- For each report, inspect the two conflicting access stacks and the synchronization annotations. Determine whether the accesses should be ordered by a lock, atomic operation, reference-count rule, interrupt masking, or another documented mechanism.
What a KCSAN report does not tell you
KCSAN identifies conflicting unsynchronized accesses; it does not automatically decide which access is semantically correct. Some intentionally racy patterns are valid when they use an accepted publication or atomicity scheme, and those cases may require annotations or a redesign so the intended ordering is explicit. Conversely, a race that never appears in a particular run remains possible because sampling and scheduling are probabilistic.
Interpreting the documented overhead figures
Linux kernel documentation from 2021, describing the v5.12-era KCSAN implementation, reported a 5.0x slowdown with the default KCSAN configuration. The same documentation measured 2.8x slowdown from KCSAN runtime fast-path overhead alone under its documented high-skip tuning. Those are configuration- and workload-specific measurements, not a universal benchmark for current kernels. Use them to budget a dedicated test kernel, not to rank KCSAN against KASAN or lockdep on a different machine.
lockdep in detail
How dependency validation finds deadlock risk
When a task acquires a lock, lockdep records the lock class and the locks already held. It does the same for every release. Across all observed paths, it constructs a directed dependency graph. A cycle indicates that different paths can impose incompatible ordering; lockdep reports the chain that creates the cycle, often showing the first path and the path that completes it.
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Enable and exercise lockdep
The driver debugging guide lists CONFIG_DEBUG_LOCKDEP as lockdep’s prerequisite and describes runtime dependency validation. Select the lockdep debugging options required by the kernel version and architecture, build a test kernel, and run the paths that acquire the relevant locks.
- Enable
CONFIG_DEBUG_LOCKDEPand the associated lock debugging configuration offered by your kernel tree. - Boot the instrumented kernel and reproduce the suspected lock sequence, including error paths and teardown paths where lock nesting often differs.
- Read the report’s lock classes, acquisition stacks, and “held here” versus “acquired here” sections. Write down the intended global order.
- Make every path follow that order, or remove the unnecessary nesting. For a recursive-lock warning, decide whether recursion is intended and use the kernel’s documented primitive or redesign the call graph.
- Repeat the workload with lockdep enabled and with the relevant KUnit or kselftest cases so that both normal and failure paths are covered.
When lockdep is not enough
A lockdep-clean run does not rule out a data race protected by no lock, a missing memory barrier, or an incorrect atomic protocol. It also does not identify an out-of-bounds access that happens while all locks are acquired correctly. Use KCSAN for conflicting memory accesses and KASAN for invalid addresses or lifetimes.
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Which detector should you run first?
Use this symptom-to-tool flow:
- Invalid address or object lifetime: start with KASAN. Select generic mode for broad architecture support, or an arm64 tag-based mode when its lower-overhead testing or MTE deployment goal fits.
- Conflicting unsynchronized accesses: start with KCSAN. Repeat the workload because sampling can miss a race.
- Lock inversion, recursive acquisition, or a deadlock warning: start with lockdep and exercise all lock-taking paths.
- Unclear failure class: choose the smallest combination covering the hypotheses. KASAN plus lockdep is a common driver bring-up pairing when memory lifetime and locking are both suspect; KCSAN plus lockdep is useful for concurrency-heavy code where races and lock ordering may interact.
Compare candidate setups on four axes: the bug class covered, architecture and compiler support, acceptable memory and runtime overhead, and how easily the detector fits the project’s KUnit or kselftest workload.
Can KCSAN, KASAN, and lockdep run together?
They can be combined as complementary instrumentation, but “together” should mean a deliberate test configuration rather than blindly enabling every option. A single build may face architecture, compiler, memory, or feature interactions, and the aggregate overhead can make timing-sensitive tests less representative. In practice, maintain focused configurations:
| Configuration | Use it when | What to watch |
|---|---|---|
| KASAN only | The reproducer is a clear bounds or lifetime failure | Generic KASAN memory and performance cost; tag-mode architecture requirements |
| KCSAN only | The issue is an intermittent race | Sampling probability, delay settings, and repeated execution |
| lockdep only | The report or design points to lock ordering | Coverage of every acquisition path and lock-class interpretation |
| KASAN + lockdep | A driver bring-up involves pointer lifetime and lock nesting | Combined slowdown and altered timing; keep the reproducer deterministic |
| KCSAN + lockdep | Concurrency-heavy code has both unsynchronized accesses and complex locking | Sampling overhead and the possibility that instrumentation changes scheduling |
Run the same test case in a focused detector build after a combined run. This isolates whether a fix addressed the intended defect and prevents a second detector’s timing changes from masking the original behavior.
Use KUnit and kselftest to make reports repeatable
The Linux testing guidance recommends pairing these detectors with kernel tests. KUnit is suited to in-kernel white-box tests that can call internal functions and check precise state transitions. kselftest is suited to userspace-visible system behavior, such as device, filesystem, networking, or syscall interactions.
A KUnit workflow
- Write a focused test around the suspected allocation, shared state, or lock sequence.
- Run that test on a detector-enabled kernel, keeping the test input and iteration count fixed.
- Capture the detector report together with the KUnit assertion output.
- After the fix, retain the test as a regression case and run it in the detector configuration in continuous integration where the cost is acceptable.
A kselftest workflow
- Choose the userspace scenario that drives the failing kernel path.
- Run it repeatedly on the instrumented kernel, collecting both standard test results and kernel logs.
- For KCSAN, vary repetition and sampling controls; for lockdep, include alternate teardown and error paths; for KASAN, include stress that reaches allocation and free boundaries.
- Re-run the same test on a normal kernel to separate detector findings from unrelated test failures.
Common misclassifications and the right response
“KASAN found it, so it must be a KASAN bug”
KASAN is the messenger for the invalid access. The underlying cause may be a missing lock, a race in reference counting, or an incorrect error path. Use the allocation/free stacks to establish the lifetime failure, then add KCSAN or lockdep if the ownership transition is concurrent or lock-protected.
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KCSAN reports conflicting memory accesses, not necessarily a lock-order cycle. The accesses may need an atomic operation, a memory barrier, a different ownership protocol, or no shared state at all. Run lockdep when multiple locks are involved or when the fix changes lock nesting.
“No lockdep warning means the subsystem cannot deadlock”
Lockdep only analyzes relationships observed by the test. An unexercised path, a lock class that is incorrectly identified, or a deadlock mechanism outside lock ordering can escape detection. Expand path coverage and inspect the design rather than treating silence as a proof.
“A clean KCSAN run proves the race is gone”
Sampling means a race can be missed. Confirm a fix with repeated executions, varied scheduling, and a regression test. If the report disappears only under one watchpoint setting, continue testing.
Planning overhead, architecture, and compiler constraints
Detector choice is also a build and resource decision:
- KCSAN: requires GCC 11+ or Clang 11+ according to the cited documentation, and its watchpoint sampling controls change both runtime cost and observation probability.
- KASAN: generic mode covers many architectures but has significant overhead; software and hardware tag-based modes in the cited v6.6 documentation are arm64-only, with hardware mode additionally relying on MTE.
- lockdep: is intended for debugging lock relationships and adds runtime graph-validation work. Its value depends on exercising the relevant paths, not on a universal speed ranking.
There is no authoritative apples-to-apples speed ranking across all three tools. Their instrumentation costs differ by workload, architecture, compiler, kernel version, and enabled options. Select the detector for coverage first, then decide whether to run it in every CI job, in scheduled stress jobs, or only in a dedicated debugging pipeline.
A practical triage checklist
- What is the earliest reliable symptom: an invalid address, a conflicting access, or a lock warning?
- Which object, shared variable, or lock is named in the report?
- Is the test running on an architecture and compiler supported by the selected mode?
- For KASAN, is the report about bounds or lifetime, and do the allocation/free stacks agree?
- For KCSAN, were the workload, iteration count, watchpoint frequency, and delay recorded?
- For lockdep, does the reported dependency cycle match the intended global lock order?
- Was the path exercised through normal operation, error handling, and teardown?
- Can a focused KUnit or kselftest case reproduce the finding after a reboot into the detector kernel?
- After the fix, did the same test pass repeatedly, and did a second detector reveal a related defect?
Bottom line
KASAN is the first choice for invalid memory accesses, KCSAN for data races, and lockdep for lock-ordering and deadlock risks. Treat them as different lenses on the same kernel: a memory failure may have a concurrency cause, and a concurrency fix may alter lock ordering. Use focused detector builds, repeatable KUnit or kselftest workloads, and the smallest combination that covers the failure hypotheses.
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