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kern/51412: Syscall I/O race condition leads to deadlock and lost interrupts

>Number:         51412
>Category:       kern
>Synopsis:       Syscall I/O race condition leads to deadlock and lost interrupts
>Confidential:   no
>Severity:       critical
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Aug 12 05:40:00 +0000 2016
>Originator:     Ryan Brackenbury
>Release:        NetBSD-7.0.1
NetBSD 7.0.1 (GENERIC.201605221355Z) amd64
Making simultaneous reads of kernel parameters can cause deadlock of some kernel I/O. Programs trying to read/write to the same kernel parameters after this time are blocked, and hang on I/O indefinitely - these show 'D' in the run status field in top/ps aux. 

Control-C's are lost after this point, and other interrupts become unpredictable or unresponsive. If 'zombie' processes waiting on kern I/O build up, this further causes a system crash.

In my situation, having two seperate users simultaneously run `envstat` caused the kern enter this state, and to drop Control-C's. The system became entirely unresponsive and required a hard reboot.
Run two copies of a program that access kernel parameters at the same time, and the will cause a deadlock. May require repeating a number of times until race occurs.

Worst Case: eg., while [ 1 ]; do sysctl hw & envstat; done
Sometimes running `sysctl -a` will unblock the kern I/O (after a few tries), and the system will return back to normal. This might point to some lock in the kernel being faulty, but I have not investigated further. 

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