macOS kernel return 39 (KERN_LOCK_UNSTABLE): meaning and diagnosis

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: The thread holding a Mach lock terminated before releasing it, leaving ownership unstable.

Could be also:
Constant	Type	OS
ERROR_HANDLE_DISK_FULL	Win32 error	Windows
ippStsEvenMedianWeight	Intel Ipp Status	Any
ENOTEMPTY	errno	Linux
EDESTADDRREQ	errno	Mac
EL3HLT	errno	Solaris
ENOLCK	errno	Windows
RDR_FILE_SYSTEM	BugCheck Code	Windows

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KERN_LOCK_SET_DESTROYED	KERN_LOCK_OWNED

KERN_LOCK_UNSTABLE

An ownership failure caused by thread termination

KERN_LOCK_UNSTABLE is more serious than ordinary contention. XNU uses it when the thread that held a lock terminated without releasing it, leaving the lock ownership state unusable for the attempted operation.

A blind retry can hide the important fact that a thread died inside a critical section. The useful root-cause evidence is the terminating thread, the protected state, and the operation in progress while the lock was held.

What to inspect

Record the lock or lock-set identity and the terminating thread last activity.
Determine whether thread exit was expected, cancelled, aborted, or caused by an exception.
Inspect partially updated data guarded by the lock.
Review shutdown ordering that allowed the holder to disappear before cleanup completed.

References

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