Re: pg_jobc going negative?

[Kamil Rytarowski <kamil%netbsd.org@localhost>]

On 09.06.2020 08:38, Maxime Villard wrote:
>> Should we allow pg_jobc going negative?
> 
> I don't think so, the code is not designed to expect negative values.

Here is the simplest reproducer crashing the kernel on negative pg_jobc:

http://netbsd.org/~kamil/ptrace/pg_jobc-crash.c

On 09.06.2020 08:38, Maxime Villard wrote:
> In short, (1) my understanding of it is that the code is not designed to
> expect negative values, and (2) since I added the KASSERTs 10/11 of the
> random bugs didn't trigger. Big signs the bug is indeed related to
> refcounting.
>
> It would be nice if someone with better understanding than me of the lwp
> group stuff could have a look, though.


Generally, pg_jobc looks like a ref counting mechanism. FreeBSD reworked
the code and added a dedicated pgrp mutex.

I don't know which path is the best for us, especially regarding the SMP
properties.

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