Relatively deterministic panic with sendfile(2) when running tests in the sxlock code

[Enji Cooper (yaneurabeya)]

> On Oct 14, 2018, at 7:25 PM, Gleb Smirnoff <glebius at freebsd.org> wrote:
> 
>  Hi Enji,
> 
> On Sun, Oct 14, 2018 at 06:51:42PM -0700, Enji Cooper (yaneurabeya) wrote:
> E> Hi,
> E> 	I’m seeing a semi-deterministic panic on 12.0-ALPHA9 related to sendfile(2) when running sendfile_test on the host: https://pastebin.com/raw/6Y7xg0ki; it looks like it’s crashing in the sxlock code when calling sblock on a sockbuf. Are there any commands in gdb you would like me to run to display lock state?
> E> 	Repro:
> E>
> E> mkdir /path/to/git/checkout
> E> cd /path/to/git/checkout
> E> git clone https://github.com/ngie-eign/freebsd/tree/sendfile_tests .
> E> git checkout sendfile_tests
> E> (cd lib/libc/tests/sys/; make obj; make; sudo make install)
> E> kyua test -k /usr/tests/lib/libc/sys/Kyuafile sendfile_test
> 
> I'd like to reproduce it myself, but looks like URL is
> wrong:
> 
> glebius at erla:/usr/src:|>git clone https://github.com/ngie-eign/freebsd/tree/sendfile_tests
> Клонирование в «sendfile_tests»…
> fatal: repository 'https://github.com/ngie-eign/freebsd/tree/sendfile_tests/' not found

Mea culpa. It should be:

$ git clone https://github.com/ngie-eign/freebsd.git <https://github.com/ngie-eign/freebsd.git> .

Another note is that I’m running GENERIC-NODEBUG, not GENERIC-DEBUG.

I suspect that it’s crashing on :hdtr_negative_bad_pointers or : s_negative_not_descriptor, because the other items don’t seem terribly plausible.

The test case (source) can be found here: https://github.com/ngie-eign/freebsd/blob/95b96470a3a0270c36c4e7fb5eedc150fe124fac/lib/libc/tests/sys/sendfile_test.c

Thanks!
-Enji
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