Re: RFR(S) 8249293: Unsafe stackwalk in VM_GetOrSetLocal::doit_prologue()

Mon, 10 Aug 2020 19:03:10 -0700 

Hi Serguei,

On 11/08/2020 3:21 am, serguei.spit...@oracle.com wrote:

Hi Richard and David,

The implementation looks good to me.
But I do not understand what the test is doing with all this counters and recursions. 

For instance, these fragments:

   86         recursions = 0;
   87         try {
   88             recursiveMethod(1<<20);
   89         } catch (StackOverflowError e) {
   90             msg("Caught StackOverflowError as expected");
   91         }
   92         int target_depth = recursions-100;    // spaces are missed around 
the '-' sigh

It is not obvious that the 'recursion' is updated in the recursiveMethod.
I would suggestto make it more explicit:
    recursiveMethod(M);
    int target_depth = M - 100;

Then the variable 'recursions' can be removed or become local.
The recursiveMethod takes in the maximum recursions to try and updates the recursions variable to record how many recursions were possible - so: 

target_depth = <actual recursions> - 100;
Possibly recursiveMethod could return the actual recursions instead of using the global variables? 

David
-----


This method will be:

   47     private static final int M = 1 << 20;
  ...
  121     public long recursiveMethod(int depth) {
  123         if (depth == 0) {
  124             notifyAgentToGetLocalAndWaitShortly(M - 100, 
waitTimeInNativeAfterNotify);
  126         } else {
  127             recursiveMethod(--depth);
  128         }
  129     }


At least, he test is missing the comments explaining all these.

Thanks,
Serguei



On 8/9/20 22:35, David Holmes wrote:

Hi Richard,

On 31/07/2020 5:28 pm, Reingruber, Richard wrote:

Hi,

I rebase the fix after JDK-8250042.

New webrev: http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.2/


The general fix for this seems good. A minor nit:

 588     if (!is_assignable(signature, ob_k, Thread::current())) {
You know that the current thread is the VMThread so can use VMThread::vm_thread(). 

Similarly for this existing code:

 694     Thread* current_thread = Thread::current();

---
Looking at the test code ... I'm less clear on exactly what is happening and the use of spin-waits raises some red-flags for me in terms of test reliability on different platforms. The "while (--waitCycles > 0)" loop in particular offers no certainty that the agent thread is executing anything in particular. And the use of the spin_count as a guide to future waiting time seems somewhat arbitrary. In all seriousness I got a headache trying to work out how the test was expecting to operate. Some parts could be simplified using raw monitors, I think. But there's no sure way to know the agent thread is in the midst of the stackwalk when the target thread wants to leave the native code. So I understand what you are trying to achieve here, I'm just not sure how reliably it will actually achieve it.
test/hotspot/jtreg/serviceability/jvmti/GetLocalVariable/libGetLocalWithoutSuspendTest.cpp 

 32 static volatile jlong spinn_count     = 0;

Using a 64-bit counter seems like it will be a problem on 32-bit systems.

Should be spin_count not spinn_count.
 36 // Agent thread waits for value != 0, then performas the JVMTI call to get local variable. 

typo: performas

Thanks,
David
-----


Thanks, Richard.

-----Original Message-----
From: serviceability-dev <serviceability-dev-r...@openjdk.java.net> On Behalf Of Reingruber, Richard 
Sent: Montag, 27. Juli 2020 09:45
To: serguei.spit...@oracle.com; serviceability-dev@openjdk.java.net
Subject: [CAUTION] RE: RFR(S) 8249293: Unsafe stackwalk in VM_GetOrSetLocal::doit_prologue() 

Hi Serguei,

   > I tested it on Linux and Windows but not yet on MacOS.

The test succeeded now on all platforms.

Thanks, Richard.

-----Original Message-----
From: Reingruber, Richard
Sent: Freitag, 24. Juli 2020 15:04
To: serguei.spit...@oracle.com; serviceability-dev@openjdk.java.net
Subject: RE: RFR(S) 8249293: Unsafe stackwalk in VM_GetOrSetLocal::doit_prologue() 

Hi Serguei,


The fix itself looks good to me.


thanks for looking at the fix.


I still need another look at new test.
Could you, please, convert the agent of new test to C++?
It will make it a little bit simpler.


Sure, here is the new webrev.1 with a C++ version of the test agent:

http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.1/

I tested it on Linux and Windows but not yet on MacOS.

Thanks,
Richard.

-----Original Message-----
From: serguei.spit...@oracle.com <serguei.spit...@oracle.com>
Sent: Freitag, 24. Juli 2020 00:00
To: Reingruber, Richard <richard.reingru...@sap.com>; serviceability-dev@openjdk.java.net Subject: Re: RFR(S) 8249293: Unsafe stackwalk in VM_GetOrSetLocal::doit_prologue() 

Hi Richard,

Thank you for filing the CR and taking care about it!
The fix itself looks good to me.
I still need another look at new test.
Could you, please, convert the agent of new test to C++?
It will make it a little bit simpler.

Thanks,
Serguei


On 7/20/20 01:15, Reingruber, Richard wrote:

Hi,
please help review this fix for VM_GetOrSetLocal. It moves the unsafe stackwalk from the vm operation prologue before the safepoint into the doit() method executed at the safepoint.
Webrev: http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.0/index.html 
Bug: https://bugs.openjdk.java.net/browse/JDK-8249293
According to the JVMTI spec on local variable access it is not required to suspend the target thread T [1]. The operation will simply fail with JVMTI_ERROR_NO_MORE_FRAMES if T is executing bytecodes. It will succeed though if T is blocked because of synchronization or executing some native 
code.
The issue is that in the latter case the stack walk in VM_GetOrSetLocal::doit_prologue() to prepare the access to the local variable is unsafe, because it is done before the safepoint and it races with T returning to execute bytecodes making its stack not walkable. The included test shows that 
this can crash the VM if T wins the race.

Manual testing:
    - new test test/hotspot/jtreg/serviceability/jvmti/GetLocalVariable/GetLocalWithoutSuspendTest.java 
    - test/hotspot/jtreg/vmTestbase/nsk/jvmti
    - test/hotspot/jtreg/serviceability/jvmti
Nightly regression tests @SAP: JCK and JTREG, also in Xcomp mode, SPECjvm2008, SPECjbb2015, Renaissance Suite, SAP specific tests with fastdebug and release builds on all platforms 

Thanks, Richard.
[1] https://docs.oracle.com/en/java/javase/14/docs/specs/jvmti.html#local

Reply via email to