[15] RFR: 8246381: VM crashes with "Current BasicObjectLock* below than low_mark"

Jamsheed C M jamsheed.c.m at oracle.com
Thu Jul 16 14:49:48 UTC 2020

Hi Coleen,

Thank you for the review.

Best regards,


On 16/07/2020 20:13, coleen.phillimore at oracle.com wrote:
> Thanks to David's description of the problem and the fix, this makes 
> sense to me now.
> I don't like it and we should revisit async exceptions for all the 
> other problems it causes, but this change looks safe and good.
> thanks,
> Coleen
> On 7/16/20 3:00 AM, Jamsheed C M wrote:
>> Hi all,
>> could i get another review?
>> Best regards,
>> Jamsheed
>> On 16/07/2020 06:37, David Holmes wrote:
>>> Hi Jamsheed,
>>> tl;dr version: fix looks good. Thanks for working through things 
>>> with me on this one.
>>> Long version ... for the sake of other reviewers (and myself) I'm 
>>> going to walk through the problem scenario and how the fix addresses 
>>> it, because the bug report is long and confusing and touches on a 
>>> number of different issues with async exception handling.
>>> We are dealing with the code generated for Java method entry, and in 
>>> particular for a synchronized Java method. We do a lot of things in 
>>> the entry code before we actually lock the monitor and jump to the 
>>> Java method. Some of those things include method profiling and the 
>>> counter overflow check for the JIT. If an exception is thrown at 
>>> this point, the logic to remove the activation would unlock the 
>>> monitor - which we haven't actually locked yet! So we have the 
>>> do_not_unlock_if_synchronized flag which is stored in the current 
>>> JavaThread. We set that flag true so that if any exceptions result 
>>> in activation removal, the removal logic won't try to unlock the 
>>> monitor. Once we're ready to lock the monitor we set the flag back 
>>> to false (note there is an implicit assumption here that monitor 
>>> locking can never raise an exception).
>>> The problem arises with async exceptions, or more specifically the 
>>> async exception that is raised due to an "unsafe access error". This 
>>> is where a memory-mapped ByteBuffer causes an access violation 
>>> (SEGV) due to a bad pointer. The signal handler simply sets a flag 
>>> to indicate we encountered an "unsafe access error", adjusts the BCI 
>>> to the next instruction and allows execution to proceed at the next 
>>> instruction. It is then expected that the runtime will "soon" notice 
>>> this pending unsafe access error and create and throw the 
>>> InternalError instance that indicates the ByteBuffer operation 
>>> failed. This requires executing Java code.
>>> One of the places that checks for that pending unsafe access error 
>>> is in the destructor of the JRT_ENTRY wrapper that is used for the 
>>> method profiling and counter overflow checking. This occurs whilst 
>>> the do_not_unlock_if_synchronized flag is true, so the resulting 
>>> InternalError won't result in an attempt to unlock the not-locked 
>>> monitor.
>>> The problem is that creating the InternalError executes Java code - 
>>> it calls constructors, which call methods etc. And some of those 
>>> methods are synchronized. So the method entry logic for such a call 
>>> will set do_not_unlock_if_synchronized to true, perform all the 
>>> preamble related to the call, then set do_not_unlock_if_synchronized 
>>> to false, lock the monitor and make the call. When construction 
>>> completes the InternalError is thrown and we remove the activation 
>>> for the method we had originally started to call. But now the 
>>> do_not_unlock_if_synchronized flag has been reset to false by the 
>>> nested Java method call, so we do in fact try to unlock a monitor 
>>> that was never locked, and things break.
>>> This nesting problem is well known and we have a mechanism for 
>>> dealing with - the UnlockFlagSaver. The actual logic executed for 
>>> profiling methods and doing the counter overflow check contains the 
>>> requisite UnlockFlagSaver to avoid the problem just outlined. 
>>> Unfortunately the async exception is processed in the JRT_ENTRY 
>>> wrapper, which is outside the scope of those UnlockFlagSaver helpers 
>>> and so they don't help in this case.
>>> So the fix is to "simply" move the UnlockFlagSaver deeper into the 
>>> call stack to the code that actually does the async exception 
>>> processing:
>>>  void JavaThread::check_and_handle_async_exceptions(bool 
>>> check_unsafe_error) {
>>> +   // May be we are at method entry and requires to save do not 
>>> unlock flag.
>>> +   UnlockFlagSaver fs(this);
>>> so now after the InternalError has been created and thrown we will 
>>> restore the original value of the do_not_unlock_if_synchronized flag 
>>> (false) and so the InternalError will not cause activation removal 
>>> to attempt to unlock the not-locked monitor.
>>> The scope of the UnlockFlagSaver could be narrowed to the actual 
>>> logic for processing the unsafe access error, but it seems fine at 
>>> method scope.
>>> A second fix is that the overflow counter check had an assertion 
>>> that it was not executed with any pending exceptions. But that 
>>> turned out to be false for reasons I can't fully explain, but it 
>>> again appears to relate to a pending async exception being installed 
>>> prior to the method call - and seems related to the two referenced 
>>> JVM TI functions. The simple solution here is to delete the 
>>> assertion and to check for pending exceptions on entry to the code 
>>> and just return immediately. The JRT_ENTRY destructor will see the 
>>> pending exception and propagate it.
>>> Cheers,
>>> David
>>> On 16/07/2020 9:50 am, David Holmes wrote:
>>>> Hi Jamsheed,
>>>> On 16/07/2020 8:16 am, Jamsheed C M wrote:
>>>>> (Thank you Dean, adding serviceability team as this issue involves 
>>>>> JVMTI features PopFrame, EarlyReturn features)
>>>> It is not at all obvious how your proposed fix impacts the JVM TI 
>>>> features.
>>>>> JBS entry: https://bugs.openjdk.java.net/browse/JDK-8246381
>>>>> (testing: mach5, tier1-5 links in JBS)
>>>>> Best regards,
>>>>> Jamsheed
>>>>> On 15/07/2020 21:25, Jamsheed C M wrote:
>>>>>> Hi,
>>>>>> Async handling at method entry requires it to be aware of 
>>>>>> synchronization(like whether it is doing async handling before 
>>>>>> lock acquire or after)
>>>>>> This is required as exception handler rely on this info for 
>>>>>> unlocking.  Async handling code never had this special condition 
>>>>>> handled and it worked most of the time as we were using biased 
>>>>>> locking which got disabled by [1]
>>>>>> There was one other issue reported in similar time[2]. This issue 
>>>>>> got triggered in test case by [3], back to back extra safepoint 
>>>>>> after suspend and TLH for ThreadDeath. So in this setup both 
>>>>>> PopFrame request and Thread.Stop request happened together for 
>>>>>> the test scenario and it reached java method entry with 
>>>>>> pending_exception set.
>>>>>> I have done a partial fix for the issue, mainly to handle 
>>>>>> production mode crash failures(do not unlock flag related ones)
>>>>>> Fix detail:
>>>>>> 1) I save restore the "do not unlock" flag in async handling.
>>>> Sorry but you completely changed the fix compared to what we 
>>>> discussed and what I pre-reviewed! What happened to changing from 
>>>> JRT_ENTRY to JRT_ENTRY_NOASYNC? It is going to take me a lot of 
>>>> time and effort to determine that this save/restore of the "do not 
>>>> unlock flag" is actually correct and valid!
>>>>>> 2) Return for floating pending exception for some cases(PopFrame, 
>>>>>> Early return related). This is debug(JVMTI) feature and floating 
>>>>>> exception can get cleaned just like that in present compiler 
>>>>>> request and deopt code.
>>>> What part of the change addresses this?
>>>> Thanks,
>>>> David
>>>> -----
>>>>>> webrev :http://cr.openjdk.java.net/~jcm/8246381/webrev.02/
>>>>>> There are more problems in these code areas, like we clear all 
>>>>>> exceptions in compilation request path(interpreter,c1), as well 
>>>>>> as deoptimization path.
>>>>>> All these un-handled cases will be separately handled by 
>>>>>> https://bugs.openjdk.java.net/browse/JDK-8249451
>>>>>> Request for review.
>>>>>> Best regards,
>>>>>> Jamsheed
>>>>>> [1]https://bugs.openjdk.java.net/browse/JDK-8231264 
>>>>>> <https://bugs.openjdk.java.net/browse/JDK-8231264>
>>>>>> [2] https://bugs.openjdk.java.net/browse/JDK-8246727
>>>>>> [3] https://bugs.openjdk.java.net/browse/JDK-8221207

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