Linux signals

[David Holmes]

Hi Walt,

On 9/01/2021 2:22 am, Walt Drummond wrote:
> Hi David --
> 
> Sorry, I've come to appreciate the confusion this causes; I can't tell
> if by 'thread' you mean a java thread or a POSIX thread. :)  To help
> punch through some of that, I'm going to be a bit pedantic, apologies
> in advance.

In hotspot there is a 1:1 correspondence between a Java thread and 
OS/Posix thread.

> When the kernel delivers a signal to a process/thread (in the Linux
> sense of the term), it places some data on the process stack.  That
> data includes the parameters to the POSIX signal hander (signal
> number, a siginfo_t, and a ucontext_t).  Above that in the stack, the
> kernel also places FPU state and, on AMD64, extended state
> information.  My question is does the JVM directly inspect the JVM
> program stack when running the POSIX signal handler(s).  My current
> understanding is that it does not and only relies on the POSIX signals
> APIs and data structures from libc.

As I stated before the JVM signal handler may inspect the stack, or it 
may not, depending on the nature of the signal it is processing - crash 
reporting can do a lot of things from the signal handler. You can see 
the main signal handler here:

https://github.com/openjdk/jdk/blob/master/src/hotspot/os/posix/signals_posix.cpp#L525

What is the context for asking the question?

Cheers,
David
-----


> Thanks, and sorry for the confusion.
> 
> On Thu, Jan 7, 2021 at 8:30 PM David Holmes <david.holmes at oracle.com> wrote:
>>
>> Hi Walt,
>>
>> On 8/01/2021 7:18 am, Walt Drummond wrote:
>>> Hi --
>>>
>>> I'm chasing down some information about how the JVM interacts with
>>> signals on Linux.  Specifically I need to understand if any of the
>>> Java runtime attempts to directly inspect the system stack when in a
>>> POSIX signal handler.  My survey of the code doesn't seem to show any
>>> such uses, and in fact it looks like the JVM relies only on the POSIX
>>> signals API but I figured I'd ask the experts for confirmation.
>>
>> I'm not quite sure what you are asking with regards to the "system
>> stack". The hotspot signal handler does a range of things to try and
>> determine what the signal might mean - e.g. whether it is a true error
>> or an internal use of signals that we can recover from. That may involve
>> examining the thread stack.
>>
>> Cheers,
>> David
>> -----
>>
>>> Thanks.
>>> --Walt
>>>