[Linaro-mm-sig] Re: [PATCH v18 00/42] DEPT(DEPendency Tracker)

On Fri, Dec 05, 2025 at 04:18:13PM +0900, Byungchul Park wrote:

I'm happy to see that DEPT reported real problems in practice:

https://lore.kernel.org/lkml/6383cde5-cf4b-facf-6e07-1378a485657d@I-love.SAK... https://lore.kernel.org/lkml/1674268856-31807-1-git-send-email-byungchul.par... https://lore.kernel.org/all/b6e00e77-4a8c-4e05-ab79-266bf05fcc2d@igalia.com/

I’ve added documentation describing DEPT — this should help you understand what DEPT is and how it works. You can use DEPT simply by enabling CONFIG_DEPT and checking dmesg at runtime.

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Hi Linus and folks,

Hi Linus and Ingo,

Although the DEPT tool still has areas for improvement, I am confident it employs the most appropriate method for tracking dependencies within Linux kernel.

If it is to be maintained in the source tree, which subsystem would be the most suitable for its management? Personally, I believe introducing a dedicated dependency subsystem under 'kernel/' would be ideal, though managing it within the already well-maintained scheduler or locking subsystems might be more practical.

Since DEPT tracks not only locking mechanisms but also general sleep and wake events, I have avoided placing it under the locking subsystem thus far. If you have alternative or more fitting suggestions, I would appreciate your input.

Thanks.

Byungchul

I’ve been developing a tool to detect deadlock possibilities by tracking waits/events — rather than lock acquisition order — to cover all the synchronization mechanisms. To summarize the design rationale, starting from the problem statement, through analysis, to the solution:

CURRENT STATUS

Lockdep tracks lock acquisition order to identify deadlock conditions. Additionally, it tracks IRQ state changes — via {en,dis}able — to detect cases where locks are acquired unintentionally during interrupt handling. PROBLEM

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Waits and their associated events that are never reachable can eventually lead to deadlocks. However, since Lockdep focuses solely on lock acquisition order, it has inherent limitations when handling waits and events. Moreover, by tracking only lock acquisition order, Lockdep cannot properly handle read locks or cross-event scenarios — such as wait_for_completion() and complete() — making it increasingly inadequate as a general-purpose deadlock detection tool. SOLUTION

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Once again, waits and their associated events that are never reachable can eventually lead to deadlocks. The new solution, DEPT, focuses directly on waits and events. DEPT monitors waits and events, and reports them when any become unreachable.

DEPT provides:

• Correct handling of read locks.
• Support for general waits and events.
• Continuous operation, even after multiple reports.
• Simple, intuitive annotation APIs.

There are still false positives, and some are already being worked on for suppression. Especially splitting the folio class into several appropriate classes e.g. block device mapping class and regular file mapping class, is currently under active development by me and Yeoreum Yun.

Anyway, these efforts will need to continue for a while, as we’ve seen with lockdep over two decades. DEPT is tagged as EXPERIMENTAL in Kconfig — meaning it’s not yet suitable for use as an automation tool.

However, for those who are interested in using DEPT to analyze complex synchronization patterns and extract dependency insights, DEPT would be a great tool for the purpose.

Thanks for your support and contributions to:

Harry Yoo harry.yoo@oracle.com Gwan-gyeong Mun gwan-gyeong.mun@intel.com Yunseong Kim ysk@kzalloc.com Yeoreum Yun yeoreum.yun@arm.com

FAQ

Q. Is this the first attempt to solve this problem? A. No. The cross-release feature (commit b09be676e0ff2) attempted to address it — as a Lockdep extension. It was merged, but quickly reverted, because:

While it uncovered valuable hidden issues, it also introduced false positives. Since these false positives mask further real problems with Lockdep — and developers strongly dislike them — the feature was rolled back.

Q. Why wasn’t DEPT built as a Lockdep extension? A. Lockdep is the result of years of work by kernel developers — and is now very stable. But I chose to build DEPT separately, because:

While reusing BFS(Breadth First Search) and Lockdep’s hashing is beneficial, the rest of the system must be rebuilt from scratch to align with DEPT’s wait-event model — since Lockdep was originally designed for tracking lock acquisition orders, not wait-event dependencies.

Q. Do you plan to replace Lockdep entirely? A. Not at all — Lockdep still plays a vital role in validating correct lock usage. While its dependency-checking logic should eventually be superseded by DEPT, the rest of its functionality should stay.

Q. Should we replace the dependency check immediately? A. Absolutely not. Lockdep’s stability is the result of years of hard work by kernel developers. Lockdep and DEPT should run side by side until DEPT matures.

Q. Stronger detection often leads to more false positives — which was a major pain point when cross-release was added. Is DEPT designed to handle this? A. Yes. DEPT’s simple, generalized design enables flexible reporting — so while false positives still need fixing, they’re far less disruptive than they were under the Lockdep extension, cross-release.

Q. Why not fix all false positives out-of-tree before merging? A. Since the affected subsystems span the entire kernel, like Lockdep, which has relied on annotations to avoid false positives over the last two decades, DEPT too will require the annotation efforts.

Performing annotation work within the mainline will help us add annotations more appropriately and will also make DEPT a useful tool for a wider range of users more quickly.

CONFIG_DEPT is marked EXPERIMENTAL, so it’s opt-in. Some users are already interested in using DEPT to analyze complex synchronization patterns and extract dependency insights.

Byungchul

Changes from v17:

1. Rebase on the mainline as of 2025 Dec 5.
2. Convert the documents' format from txt to rst. (feedbacked by Jonathan Corbet and Bagas Sanjaya)
3. Move the documents from 'Documentation/dependency' to 'Documentation/dev-tools'. (feedbakced by Jonathan Corbet)
4. Improve the documentation. (feedbacked by NeilBrown)
5. Use a common function, enter_from_user_mode(), instead of arch specific code, to notice context switch from user mode. (feedbacked by Dave Hansen, Mark Rutland, and Mark Brown)
6. Resolve the header dependency issue by using dept's internal header, instead of relocating 'struct llist_{head,node}' to another header. (feedbacked by Greg KH)
7. Improve page(or folio) usage type APIs.
8. Add rust helper for wait_for_completion(). (feedbacked by Guangbo Cui, Boqun Feng, and Danilo Krummrich)
9. Refine some commit messages.

Changes from v16:

1. Rebase on v6.17.
2. Fix a false positive from rcu (by Yunseong Kim)
3. Introduce APIs to set page's usage, dept_set_page_usage() and dept_reset_page_usage() to avoid false positives.
4. Consider lock_page() as a potential wait unconditionally.
5. Consider folio_lock_killable() as a potential wait unconditionally.
6. Add support for tracking PG_writeback waits and events.
7. Fix two build errors due to the additional debug information added by dept. (by Yunseong Kim)

Changes from v15:

1. Fix typo and improve comments and commit messages (feedbacked by ALOK TIWARI, Waiman Long, and kernel test robot).
2. Do not stop dept on detection of cicular dependency of recover event, allowing to keep reporting.
3. Add SK hynix to copyright.
4. Consider folio_lock() as a potential wait unconditionally.
5. Fix Kconfig dependency bug (feedbacked by kernel test rebot).
6. Do not suppress reports that involve classes even that have already involved in other reports, allowing to keep reporting.

Changes from v14:

1. Rebase on the current latest, v6.15-rc6.
2. Refactor dept code.
3. With multi event sites for a single wait, even if an event forms a circular dependency, the event can be recovered by other event(or wake up) paths. Even though informing the circular dependency is worthy but it should be suppressed once informing it, if it doesn't lead an actual deadlock. So introduce APIs to annotate the relationship between event site and recover site, that are, event_site() and dept_recover_event().
4. wait_for_completion() worked with dept map embedded in struct completion. However, it generates a few false positves since all the waits using the instance of struct completion, share the map and key. To avoid the false positves, make it not to share the map and key but each wait_for_completion() caller have its own key by default. Of course, external maps also can be used if needed.
5. Fix a bug about hardirq on/off tracing.
6. Implement basic unit test for dept.
7. Add more supports for dma fence synchronization.
8. Add emergency stop of dept e.g. on panic().
9. Fix false positives by mmu_notifier_invalidate_*().
10. Fix recursive call bug by DEPT_WARN_*() and DEPT_STOP().
11. Fix trivial bugs in DEPT_WARN_*() and DEPT_STOP().
12. Fix a bug that a spin lock, dept_pool_spin, is used in both contexts of irq disabled and enabled without irq disabled.
13. Suppress reports with classes, any of that already have been reported, even though they have different chains but being barely meaningful.
14. Print stacktrace of the wait that an event is now waking up, not only stacktrace of the event.
15. Make dept aware of lockdep_cmp_fn() that is used to avoid false positives in lockdep so that dept can also avoid them.
16. Do do_event() only if there are no ecxts have been delimited.
17. Fix a bug that was not synchronized for stage_m in struct dept_task, using a spin lock, dept_task()->stage_lock.
18. Fix a bug that dept didn't handle the case that multiple ttwus for a single waiter can be called at the same time e.i. a race issue.
19. Distinguish each kernel context from others, not only by system call but also by user oriented fault so that dept can work with more accuracy information about kernel context. That helps to avoid a few false positives.
20. Limit dept's working to x86_64 and arm64.

Changes from v13:

1. Rebase on the current latest version, v6.9-rc7.
2. Add 'dept' documentation describing dept APIs.

Changes from v12:

1. Refine the whole document for dept.

2. Add 'Interpret dept report' section in the document, using a deadlock report obtained in practice. Hope this version of document helps guys understand dept better.

   https://lore.kernel.org/lkml/6383cde5-cf4b-facf-6e07-1378a485657d@I-love.SAK... https://lore.kernel.org/lkml/1674268856-31807-1-git-send-email-byungchul.par...

Changes from v11:

1. Add 'dept' documentation describing the concept of dept.

2. Rewrite the commit messages of the following commits for using weaker lockdep annotation, for better description.

   fs/jbd2: Use a weaker annotation in journal handling cpu/hotplug: Use a weaker annotation in AP thread

   (feedbacked by Thomas Gleixner)

Changes from v10:

1. Fix noinstr warning when building kernel source.
2. dept has been reporting some false positives due to the folio lock's unfairness. Reflect it and make dept work based on dept annotaions instead of just wait and wake up primitives.
3. Remove the support for PG_writeback while working on 2. I will add the support later if needed.
4. dept didn't print stacktrace for [S] if the participant of a deadlock is not lock mechanism but general wait and event. However, it made hard to interpret the report in that case. So add support to print stacktrace of the requestor who asked the event context to run - usually a waiter of the event does it just before going to wait state.
5. Give up tracking raw_local_irq_{disable,enable}() since it totally messed up dept's irq tracking. So make it work in the same way as lockdep does. I will consider it once any false positives by those are observed again.
6. Change the manual rwsem_acquire_read(->j_trans_commit_map) annotation in fs/jbd2/transaction.c to the try version so that it works as much as it exactly needs.
7. Remove unnecessary 'inline' keyword in dept.c and add '__maybe_unused' to a needed place.

Changes from v9:

1. Fix a bug. SDT tracking didn't work well because of my big mistake that I should've used waiter's map to indentify its class but it had been working with waker's one. FYI, PG_locked and PG_writeback weren't affected. They still worked well. (reported by YoungJun)

Changes from v8:

1. Fix build error by adding EXPORT_SYMBOL(PG_locked_map) and EXPORT_SYMBOL(PG_writeback_map) for kernel module build - appologize for that. (reported by kernel test robot)
2. Fix build error by removing header file's circular dependency that was caused by "atomic.h", "kernel.h" and "irqflags.h", which I introduced - appolgize for that. (reported by kernel test robot)

Changes from v7:

1. Fix a bug that cannot track rwlock dependency properly, introduced in v7. (reported by Boqun and lockdep selftest)
2. Track wait/event of PG_{locked,writeback} more aggressively assuming that when a bit of PG_{locked,writeback} is cleared there might be waits on the bit. (reported by Linus, Hillf and syzbot)
3. Fix and clean bad style code e.i. unnecessarily introduced a randome pattern and so on. (pointed out by Linux)
4. Clean code for applying dept to wait_for_completion().

Changes from v6:

1. Tie to task scheduler code to track sleep and try_to_wake_up() assuming sleeps cause waits, try_to_wake_up()s would be the events that those are waiting for, of course with proper dept annotations, sdt_might_sleep_weak(), sdt_might_sleep_strong() and so on. For these cases, class is classified at sleep entrance rather than the synchronization initialization code. Which would extremely reduce false alarms.
2. Remove the dept associated instance in each page struct for tracking dependencies by PG_locked and PG_writeback thanks to the 1. work above.
3. Introduce CONFIG_dept_AGGRESIVE_TIMEOUT_WAIT to suppress reports that waits with timeout set are involved, for those who don't like verbose reporting.
4. Add a mechanism to refill the internal memory pools on running out so that dept could keep working as long as free memory is available in the system.
5. Re-enable tracking hashed-waitqueue wait. That's going to no longer generate false positives because class is classified at sleep entrance rather than the waitqueue initailization.
6. Refactor to make it easier to port onto each new version of the kernel.
7. Apply dept to dma fence.
8. Do trivial optimizaitions.

Changes from v5:

1. Use just pr_warn_once() rather than WARN_ONCE() on the lack of internal resources because WARN_*() printing stacktrace is too much for informing the lack. (feedback from Ted, Hyeonggon)
2. Fix trivial bugs like missing initializing a struct before using it.
3. Assign a different class per task when handling onstack variables for waitqueue or the like. Which makes dept distinguish between onstack variables of different tasks so as to prevent false positives. (reported by Hyeonggon)
4. Make dept aware of even raw_local_irq_*() to prevent false positives. (reported by Hyeonggon)
5. Don't consider dependencies between the events that might be triggered within __schedule() and the waits that requires __schedule(), real ones. (reported by Hyeonggon)
6. Unstage the staged wait that has prepare_to_wait_event()'ed *and* yet to get to __schedule(), if we encounter __schedule() in-between for another sleep, which is possible if e.g. a mutex_lock() exists in 'condition' of ___wait_event().
7. Turn on CONFIG_PROVE_LOCKING when CONFIG_DEPT is on, to rely on the hardirq and softirq entrance tracing to make dept more portable for now.

Changes from v4:

1. Fix some bugs that produce false alarms.
2. Distinguish each syscall context from another *for arm64*.
3. Make it not warn it but just print it in case dept ring buffer gets exhausted. (feedback from Hyeonggon)
4. Explicitely describe "EXPERIMENTAL" and "dept might produce false positive reports" in Kconfig. (feedback from Ted)

Changes from v3:

1. dept shouldn't create dependencies between different depths of a class that were indicated by *_lock_nested(). dept normally doesn't but it does once another lock class comes in. So fixed it. (feedback from Hyeonggon)
2. dept considered a wait as a real wait once getting to __schedule() even if it has been set to TASK_RUNNING by wake up sources in advance. Fixed it so that dept doesn't consider the case as a real wait. (feedback from Jan Kara)
3. Stop tracking dependencies with a map once the event associated with the map has been handled. dept will start to work with the map again, on the next sleep.

Changes from v2:

1. Disable dept on bit_wait_table[] in sched/wait_bit.c reporting a lot of false positives, which is my fault. Wait/event for bit_wait_table[] should've been tagged in a higher layer for better work, which is a future work. (feedback from Jan Kara)
2. Disable dept on crypto_larval's completion to prevent a false positive.

Changes from v1:

1. Fix coding style and typo. (feedback from Steven)
2. Distinguish each work context from another in workqueue.
3. Skip checking lock acquisition with nest_lock, which is about correct lock usage that should be checked by lockdep.

Changes from RFC(v0):

1. Prevent adding a wait tag at prepare_to_wait() but __schedule(). (feedback from Linus and Matthew)
2. Use try version at lockdep_acquire_cpus_lock() annotation.
3. Distinguish each syscall context from another.

Byungchul Park (41): dept: implement DEPT(DEPendency Tracker) dept: add single event dependency tracker APIs dept: add lock dependency tracker APIs dept: tie to lockdep and IRQ tracing dept: add proc knobs to show stats and dependency graph dept: distinguish each kernel context from another dept: distinguish each work from another dept: add a mechanism to refill the internal memory pools on running out dept: record the latest one out of consecutive waits of the same class dept: apply sdt_might_sleep_{start,end}() to wait_for_completion()/complete() dept: apply sdt_might_sleep_{start,end}() to swait dept: apply sdt_might_sleep_{start,end}() to waitqueue wait dept: apply sdt_might_sleep_{start,end}() to hashed-waitqueue wait dept: apply sdt_might_sleep_{start,end}() to dma fence dept: track timeout waits separately with a new Kconfig dept: apply timeout consideration to wait_for_completion()/complete() dept: apply timeout consideration to swait dept: apply timeout consideration to waitqueue wait dept: apply timeout consideration to hashed-waitqueue wait dept: apply timeout consideration to dma fence wait dept: make dept able to work with an external wgen dept: track PG_locked with dept dept: print staged wait's stacktrace on report locking/lockdep: prevent various lockdep assertions when lockdep_off()'ed dept: add documents for dept cpu/hotplug: use a weaker annotation in AP thread dept: assign dept map to mmu notifier invalidation synchronization dept: assign unique dept_key to each distinct dma fence caller dept: make dept aware of lockdep_set_lock_cmp_fn() annotation dept: make dept stop from working on debug_locks_off() dept: assign unique dept_key to each distinct wait_for_completion() caller completion, dept: introduce init_completion_dmap() API dept: introduce a new type of dependency tracking between multi event sites dept: add module support for struct dept_event_site and dept_event_site_dep dept: introduce event_site() to disable event tracking if it's recoverable dept: implement a basic unit test for dept dept: call dept_hardirqs_off() in local_irq_*() regardless of irq state dept: introduce APIs to set page usage and use subclasses_evt for the usage dept: track PG_writeback with dept SUNRPC: relocate struct rcu_head to the first field of struct rpc_xprt mm: percpu: increase PERCPU_DYNAMIC_SIZE_SHIFT on DEPT and large PAGE_SIZE

Yunseong Kim (1): rcu/update: fix same dept key collision between various types of RCU

Documentation/dev-tools/dept.rst | 778 ++++++ Documentation/dev-tools/dept_api.rst | 125 + drivers/dma-buf/dma-fence.c | 23 +- include/asm-generic/vmlinux.lds.h | 13 +- include/linux/completion.h | 124 +- include/linux/dept.h | 402 +++ include/linux/dept_ldt.h | 78 + include/linux/dept_sdt.h | 68 + include/linux/dept_unit_test.h | 67 + include/linux/dma-fence.h | 74 +- include/linux/hardirq.h | 3 + include/linux/irq-entry-common.h | 4 + include/linux/irqflags.h | 21 +- include/linux/local_lock_internal.h | 1 + include/linux/lockdep.h | 105 +- include/linux/lockdep_types.h | 3 + include/linux/mm_types.h | 4 + include/linux/mmu_notifier.h | 26 + include/linux/module.h | 5 + include/linux/mutex.h | 1 + include/linux/page-flags.h | 217 +- include/linux/pagemap.h | 37 +- include/linux/percpu-rwsem.h | 2 +- include/linux/percpu.h | 4 + include/linux/rcupdate_wait.h | 13 +- include/linux/rtmutex.h | 1 + include/linux/rwlock_types.h | 1 + include/linux/rwsem.h | 1 + include/linux/sched.h | 118 + include/linux/seqlock.h | 2 +- include/linux/spinlock_types_raw.h | 3 + include/linux/srcu.h | 2 +- include/linux/sunrpc/xprt.h | 9 +- include/linux/swait.h | 3 + include/linux/wait.h | 3 + include/linux/wait_bit.h | 3 + init/init_task.c | 2 + init/main.c | 2 + kernel/Makefile | 1 + kernel/cpu.c | 2 +- kernel/dependency/Makefile | 5 + kernel/dependency/dept.c | 3499 ++++++++++++++++++++++++++ kernel/dependency/dept_hash.h | 10 + kernel/dependency/dept_internal.h | 314 +++ kernel/dependency/dept_object.h | 13 + kernel/dependency/dept_proc.c | 94 + kernel/dependency/dept_unit_test.c | 173 ++ kernel/exit.c | 1 + kernel/fork.c | 2 + kernel/locking/lockdep.c | 33 + kernel/module/main.c | 19 + kernel/rcu/rcu.h | 1 + kernel/rcu/update.c | 5 +- kernel/sched/completion.c | 62 +- kernel/sched/core.c | 9 + kernel/workqueue.c | 3 + lib/Kconfig.debug | 48 + lib/debug_locks.c | 2 + lib/locking-selftest.c | 2 + mm/filemap.c | 38 + mm/mm_init.c | 3 + mm/mmu_notifier.c | 31 +- rust/helpers/completion.c | 5 + 63 files changed, 6602 insertions(+), 121 deletions(-) create mode 100644 Documentation/dev-tools/dept.rst create mode 100644 Documentation/dev-tools/dept_api.rst create mode 100644 include/linux/dept.h create mode 100644 include/linux/dept_ldt.h create mode 100644 include/linux/dept_sdt.h create mode 100644 include/linux/dept_unit_test.h create mode 100644 kernel/dependency/Makefile create mode 100644 kernel/dependency/dept.c create mode 100644 kernel/dependency/dept_hash.h create mode 100644 kernel/dependency/dept_internal.h create mode 100644 kernel/dependency/dept_object.h create mode 100644 kernel/dependency/dept_proc.c create mode 100644 kernel/dependency/dept_unit_test.c

base-commit: 43dfc13ca972988e620a6edb72956981b75ab6b0

2.17.1