RFR(S): 8247533: SA stack walking sometimes fails with sun.jvm.hotspot.debugger.DebuggerException: get_thread_regs failed for a lwp

[David Holmes]

On 18/06/2020 2:33 pm, Chris Plummer wrote:
> On 6/17/20 7:43 PM, David Holmes wrote:
>> Hi Chris,
>>
>> On 18/06/2020 6:34 am, Chris Plummer wrote:
>>> Hello,
>>>
>>> Please help review the following:
>>>
>>> https://bugs.openjdk.java.net/browse/JDK-8247533
>>> http://cr.openjdk.java.net/~cjplummer/8247533/webrev.00/index.html
>>>
>>> The CR contains all the needed details. Here's a summary of changes 
>>> in each file:
>>
>> The problem sounds to me like a variation of the more general problem 
>> of not ensuring a thread is kept alive whilst acting upon it. I don't 
>> know how the SA finds these references to the threads it is going to 
>> stackwalk, but is it possible to fix this via appropriate uses of 
>> ThreadsListHandle/Iterator?
> It fetches ThreadsSMRSupport::_java_thread_list.
> 
> Keep in mind that once SA attaches, nothing in the VM changes. For 
> example, SA can't create a wrapper to a JavaThread, only to have the 
> JavaThread be freed later on. It's just not possible.

Then how does it obtain a reference to a JavaThread for which the native 
OS thread id is invalid? Any thread found in _java_thread_list is either 
live or still to be started. In the latter case the JavaThread->osThread 
does not have its thread_id set yet.

David
-----

> Chris
>>
>> Cheers,
>> David
>>
>>> src/jdk.hotspot.agent/linux/native/libsaproc/LinuxDebuggerLocal.cpp
>>> src/jdk.hotspot.agent/macosx/native/libsaproc/MacosxDebuggerLocal.m
>>> src/jdk.hotspot.agent/windows/native/libsaproc/sawindbg.cpp
>>> -Instead of throwing an exception when the OS ThreadID is invalid, 
>>> print a warning.
>>>
>>> src/jdk.hotspot.agent/linux/native/libsaproc/ps_proc.c
>>> -Improve a print_debug message
>>>
>>> src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/debugger/bsd/BsdThread.java 
>>>
>>> src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/debugger/linux/LinuxThread.java 
>>>
>>> src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/debugger/windbg/amd64/WindbgAMD64Thread.java 
>>>
>>> -Deal with the array of registers read in being null due to the OS 
>>> ThreadID not being valid.
>>>
>>> src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/debugger/bsd/BsdDebuggerLocal.java 
>>>
>>> src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/debugger/linux/LinuxDebuggerLocal.java 
>>>
>>> -Fix issue with "sun.jvm.hotspot.debugger.DebuggerException" 
>>> appearing twice when printing the exception.
>>>
>>> thanks,
>>>
>>> Chris
>