[PATCH v10 00/25] DEPT(Dependency Tracker)

Byungchul Park
2023-06-26T12:43:51Z

From now on, I can work on LKML again! I'm wondering if DEPT has been
helping kernel debugging well even though it's a form of patches yet.

I'm happy to see that DEPT reports a real problem in practice. See:

https://lore.kernel.org/lkml/6383cde5-cf4b-facf-6e07-1378a485657d@I-love.SAKURA.ne.jp/#t
https://lore.kernel.org/lkml/1674268856-31807-1-git-send-email-byungchul.park@lge.com/

Nevertheless, I apologize for the lack of document. I promise to add it
before it gets needed to use DEPT's APIs by users. For now, you can use
DEPT just with CONFIG_DEPT on.

---

Hi Linus and folks,

I've been developing a tool for detecting deadlock possibilities by
tracking wait/event rather than lock(?) acquisition order to try to
cover all synchonization machanisms. It's done on v6.2-rc2.

Benifit:

0. Works with all lock primitives.
1. Works with wait_for_completion()/complete().
2. Works with 'wait' on PG_locked.
3. Works with 'wait' on PG_writeback.
4. Works with swait/wakeup.
5. Works with waitqueue.
6. Works with wait_bit.
7. Multiple reports are allowed.
8. Deduplication control on multiple reports.
9. Withstand false positives thanks to 6.
10. Easy to tag any wait/event.

Future work:

0. To make it more stable.
1. To separates Dept from Lockdep.
2. To improves performance in terms of time and space.
3. To use Dept as a dependency engine for Lockdep.
4. To add any missing tags of wait/event in the kernel.
5. To deduplicate stack trace.

How to interpret reports:

1. E(event) in each context cannot be triggered because of the
W(wait) that cannot be woken.
2. The stack trace helping find the problematic code is located
in each conext's detail.

Thanks,
Byungchul

---

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 (25):
llist: Move llist_{head,node} definition to types.h
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: Apply sdt_might_sleep_{start,end}() to
wait_for_completion()/complete()
dept: Apply sdt_might_sleep_{start,end}() to PG_{locked,writeback}
wait
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: Distinguish each syscall context from another
dept: Distinguish each work from another
dept: Add a mechanism to refill the internal memory pools on running
out
locking/lockdep, cpu/hotplus: Use a weaker annotation in AP thread
dept: Apply sdt_might_sleep_{start,end}() to dma fence wait
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: Record the latest one out of consecutive waits of the same class
dept: Make Dept able to work with an external wgen
dept: Track the potential waits of PG_{locked,writeback}

arch/arm64/kernel/syscall.c | 2 +
arch/x86/entry/common.c | 4 +
drivers/dma-buf/dma-fence.c | 5 +
include/linux/completion.h | 30 +-
include/linux/dept.h | 614 ++++++
include/linux/dept_ldt.h | 77 +
include/linux/dept_sdt.h | 66 +
include/linux/hardirq.h | 3 +
include/linux/irqflags.h | 22 +-
include/linux/llist.h | 8 -
include/linux/local_lock_internal.h | 1 +
include/linux/lockdep.h | 102 +-
include/linux/lockdep_types.h | 3 +
include/linux/mm_types.h | 3 +
include/linux/mutex.h | 1 +
include/linux/page-flags.h | 112 +-
include/linux/pagemap.h | 7 +-
include/linux/percpu-rwsem.h | 2 +-
include/linux/rtmutex.h | 1 +
include/linux/rwlock_types.h | 1 +
include/linux/rwsem.h | 1 +
include/linux/sched.h | 3 +
include/linux/seqlock.h | 2 +-
include/linux/spinlock_types_raw.h | 3 +
include/linux/srcu.h | 2 +-
include/linux/swait.h | 3 +
include/linux/types.h | 8 +
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 | 4 +
kernel/dependency/dept.c | 3167 +++++++++++++++++++++++++++
kernel/dependency/dept_hash.h | 10 +
kernel/dependency/dept_internal.h | 26 +
kernel/dependency/dept_object.h | 13 +
kernel/dependency/dept_proc.c | 93 +
kernel/exit.c | 1 +
kernel/fork.c | 2 +
kernel/locking/lockdep.c | 23 +
kernel/module/main.c | 2 +
kernel/sched/completion.c | 2 +-
kernel/sched/core.c | 9 +
kernel/workqueue.c | 3 +
lib/Kconfig.debug | 37 +
lib/locking-selftest.c | 2 +
mm/filemap.c | 18 +
mm/page_alloc.c | 3 +
50 files changed, 4459 insertions(+), 55 deletions(-)
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 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

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2.17.1