RFR (S) 8212207: runtime/InternalApi/ThreadCpuTimesDeadlock.java crashes with SEGV in pthread_getcpuclockid+0x0

[David Holmes]

Hi Thomas,

On 28/11/2018 4:30 pm, Thomas Stüfe wrote:
> Hi David,
> 
> I admit I still do not really get it..
> 
> E.g. for GC threads:
> 
> We have
> 
> static ConcurrentMarkSweepThread*
> ConcurrentMarkSweepThread::start(CMSCollector* collector);
> 
> which creates a ConcurrentMarkSweepThread and then calls
> os::create_thread() to hook it up with an OSThread and start it.
> 
> ConcurrentMarkSweepThread derives from NonJavaThread, which in its
> constructor adds the thread object to the list.
> 
> So there is a time gap where the NJT is part of the list, but
> Thread::_osthread is still NULL.
> 
> In ThreadTimesClosure::do_thread(), we call
> os::thread_cpu_time(thread)->fast_cpu_time(thread)->os::Linux::pthread_getcpuclockid(thread->osthread()->pthread_id(),
> &clockid);
> 
> Should we then not crash when dereferencing the NULL
> osthread()->pthread_id()? Why do we crash inside
> pthread_getcpuclockid?
> 
> If I look further into os::create_thread(), I see that there is
> another, smaller time gap where we create OSThread and anchor it into
> the Thread object:
> 
>    thread->set_osthread(osthread);
> 
> and then later, after pthread_create is thru, set its thread id:
> 
>      // Store pthread info into the OSThread
>      osthread->set_pthread_id(tid);
> 
> When OSThread is created, we call OSThread::pd_initialize() and set
> its _threadid to 0. We do this in the constructor, before anchoring
> the OSThread in its Thread.
> 
> So for my understanding, we should have two situations:
> 
> (1)- NJT is in list, but its _osthread member is NULL. In that case I
> would expect a different crash.
> (2)- NJT is in list, _osthread is set, but its _thread_id is NULL.

Yes both situations may be possible. In the related bug report only the 
0 thread_id was observed.

> (Modulo any concurrency issues, e.g. could the
> "thread->set_osthread(osthread);" be visible before OSThread is
> initialized?
> 
> Depending on what is happening, a fix for (1) would probably be a
> querying for osthread==NULL, a fix for (2) would be to guard os::---
> functions - well, at least os::thread_cpu_time() - to disregard
> Threads with pthread_t == 0. Both fixes seem better to me than
> querying the stacksize() when walking the list - that seems a bit
> awkward.

I find it awkward that the list is populated with partially constructed 
threads in the first place. This particular code is only interested in 
live threads that can safely be queried. The stack_size() check is a 
surrogate for "this is a live thread that can be queried".

> --
> P.s.
> 
> ConcurrentGCThread::ConcurrentGCThread() :
>    _should_terminate(false), _has_terminated(false) {
> };
> 
> I was surprised to see no invocation to the base class ctor in the
> initializer list. I was not aware that this was even possible. For
> code clearness, I would prefer the call to the base class ctor to be
> explicit.)

I assume it is implicit. But yes it should be explicit.

Cheers,
David

> --
> 
> Cheers, Thomas
> 
> 
> On Wed, Nov 28, 2018 at 3:58 AM David Holmes <david.holmes at oracle.com> wrote:
>>
>> Sorry for the delayed response.
>>
>> On 21/11/2018 3:01 pm, Kim Barrett wrote:
>>>> On Nov 20, 2018, at 3:50 AM, David Holmes <david.holmes at oracle.com> wrote:
>>>>
>>>> After discussions with Kim I've decided to split out the NJT list update into a separate RFE:
>>>>
>>>> https://bugs.openjdk.java.net/browse/JDK-8214097
>>>>
>>>> So only the change in management.cpp needs reviewing and testing.
>>>>
>>>> Updated webrev:
>>>>
>>>> http://cr.openjdk.java.net/~dholmes/8212207/webrev.v2/
>>>
>>> Looks good.
>>
>> Thanks Kim.
>>
>> I've decided to stick with this simple fix for NJTs only.
>>
>> David
>>
>>>>
>>>> Thanks,
>>>> David
>>>>
>>>> On 20/11/2018 10:01 am, David Holmes wrote:
>>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8212207
>>>>> webrev: http://cr.openjdk.java.net/~dholmes/8212207/webrev/
>>>>> There is an internal management API that reports CPU times for NonJavaThreads (NJTs). That functionality requires a valid/live target thread so that we can use its pthread_t identity to obtain its CPU clock via pthread_getcpuclockid().
>>>>> There is an iteration mechanism for NJTs in which the NJT is registered during its constructor and de-registered during its destructor. A thread that has only been constructed has not yet executed and so is not a valid target for this management API. This seems to be the cause of failures reported in this bug (and JDK-8213434). Registering a NJT only when it starts executing is an appealing fix for this, but that impacts all current users of the NJT list and straight-away causes a problem with the BarrierSet initialization logic. So I don't attempt that.
>>>>> Instead the first part of the fix is for ThreadTimesClosure::do_thread to skip threads that have not yet executed - which we can recognize by seeing an uninitialized (i.e. zero) stackbase.
>>>>> A second part of the fix, which can be deferred to a separate RFE for NJT lifecycle management if desired, tackles the problem of encountering a terminated thread during iteration - which can also lead to SEGVs. This can arise because NJT's are not actually "destructed", even if they terminate, and so they never get removed from the NJT list. Calling destructors is problematic because the code using these NJTs assume they are always valid. So the fix in this case is to move the de-registering from the NJT list out of the destructor and into the Thread::call_run() method so it is done before a thread actually terminates. This can be considered a first step in cleaning up the NJT lifecycle, where the remaining steps touch on a lot of areas and so need to be handled separately e.g. see JDK-8087340 for shutting down WorkGang GC worker threads.
>>>>> Testing: tiers 1 -3
>>>>> I should point out that I've been unable to reproduce this failure locally, even after thousands of runs. I'm hoping Zhengyu can test this in the conditions reported in JDK-8213434.
>>>>> Thanks,
>>>>> David
>>>
>>>