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[Bug 252579] fork() causes process to hang in rare circumstances.

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[Bug 252579] fork() causes process to hang in rare circumstances.

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=252579

Bug ID: 252579
Summary: fork() causes process to hang in rare circumstances.
Product: Base System
Version: CURRENT
Hardware: Any
OS: Any
Status: New
Severity: Affects Only Me
Priority: ---
Component: threads
Assignee: [hidden email]
Reporter: [hidden email]

Created attachment 221461
--> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=221461&action=edit
NSS module and test program

This bug is discovered on FreeBSD 13-CURRENT but can also be reproduced on
12.2-RELEASE. It causes a process to hung when fork(2) is called and specific
NSS (Name Switch Service) module is used.

How to reproduce:

1) Download archive in the attachment.
2) Compile NSS stub module (do not forget .1 at the end of compiled module):
cc -shared -fPIC -pthread -o nss_stub.so.1 nss_stub.c
3) Copy nss_stub.so.1 to /usr/local/lib
4) Edit /etc/nsswitch.conf and replace 'hosts: files dns' witch 'hosts: files
dns stub'
5) Compile test program: cc -o bug bug.c
6) Run it, it will hang, so even killall -9 bug won't kill it.

There is a small and unpleasant discussion on freebsd-net mailing list with
Konstantin Belousov who wanted me to reproduce this bug without editing
/etc/nsswitch.conf I think it's either impossible, because NSS system is
somehow messing with fork, or it's beyond my competence. So the provided way to
reproduce the bug is as minimal as I can get.

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[Bug 252579] fork() causes process to hang in rare circumstances.

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=252579

Jason A. Harmening <[hidden email]> changed:

What |Removed |Added
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CC| |[hidden email]

--- Comment #1 from Jason A. Harmening <[hidden email]> ---
I can reproduce this 100% of the time on a -current VM using the supplied test
code. I noticed a few things:

--for me, the parent process seems to be hanging during fork(); I see no
evidence the child process is ever spawned.

--wmesg for the process is 'umtxn', and ddb shows what looks like the main
thread attempting to take a userspace lock, going through umtxq_lock(), and
sleeping in sleepq_wait_sig()

--I tried to write a smaller test program to reproduce the failure by
simulating the locking done by the NS dispatcher and the pthread_create()
issued by the stub, but this did not reproduce the hang.

--However, if I just link the original test program against libpthread ('cc -o
bug -pthread bug.c), then I can no longer reproduce the hang. This tells me
the problem might have something to do with some bit of static umtx
initialization that happens when linking against libpthread/libthr. If this
initialization hasn't happened by the time the NS dispatcher (which loads the
stub through dlopen()) is invoked, then fork() ends up stuck in a umtx wait
that never gets signaled. It might also be related to the __isthreaded checks
made by lib/libc/net/nsdispatch.c, which smell fishy to me.

At the very least, it might be possible to make a smaller repro case by writing
a test program (that does not link libpthread) which dlopen()s a simple library
(which does link libpthread) and calls an entry point that spawns a thread.

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