Handling of USR2 signal in JVM on Linux

[Thomas Stüfe]

SIGUSR2 is used by the hotspot, internally, to implement suspend/resume. It
gets sent by hotspot via pthread_kill() to targeted threads to suspend
them. In that case it is known that the receiving thread is a valid java
thread and therefore the assert makes sense.

However, as you describe SIGUSR2 can also be sent from outside via kill(2).
In that case the receiving thread is arbitrarily chosen by the kernel. It
is not necessarily a valid java thread. In that case the VM will crash
(release) or assert (debug).

I tend to think this is an error too. Or at least in grey area. Since this
is very easy to fix in the hotspot, I'd suggest we do this.

If nobody objects, I can file an issue and prepare the patch.

Cheers, Thomas

(P.s. not C++ undefined behavior, nothing to do with C++ :-)

On Wed, Jun 15, 2022 at 12:11 PM Andrey Turbanov <turbanoff at gmail.com>
wrote:

> I mean, isn't JVM supposed to be safe? :)
> Receiving this signal _could_ happen in a real deployment. And now, as
> I can see, we have C++ undefined behaviour in release builds in this
> case. Can we consider this as a bug?
>
> Andrey Turbanov
>
> вт, 14 июн. 2022 г. в 14:46, Alan Bateman <Alan.Bateman at oracle.com>:
> >
> > On 14/06/2022 10:44, Andrey Turbanov wrote:
> > > Hello.
> > > During investigation of signal handling in JVM (for
> > > https://github.com/openjdk/jdk/pull/9100#discussion_r894992558 )
> > > I found out that sending USR2 crashes my JDK. (Linux fastdebug x64)
> > >
> > > kill -USR2 1346792
> > >
> > > # assert(thread != __null) failed: Missing current thread in SR_handler
> > > # Internal Error
> > >
> (/home/turbanoff/Projects/official_jdk/src/hotspot/os/posix/signals_posix.cpp:1600),
> > > pid=1346792, tid=1346792
> > >
> > > Full hs_err_pid1346792.log:
> > > https://gist.github.com/turbanoff/2099327ea13357a90df43a2d6b0e2e6a
> > >
> > >
> > > Is it known/expected behaviour?
> > > I found some description there
> > >
> https://docs.oracle.com/en/java/javase/11/troubleshoot/handle-signals-and-exceptions.html
> > > that USR2 is used for SUSPEND/RESUME. Is it supported by Hotspot?
> >
> > In general you have to be very careful when using signals. Yes, it can
> > easily break things and probably notice it quickly with debug builds as
> > asserts are compiled in to the builds (like the above). So I think
> > you've found the right page to read up on this. In this case, you can
> > set _JAVA_SR_SIGNUM to specify a different signal for S/R.
> >
> > -Alan
>
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