RFR: 8130212: Thread::current() might access freed memory on Solaris

Christian Thalinger christian.thalinger at oracle.com
Wed Jul 29 22:38:24 UTC 2015

> On Jul 29, 2015, at 3:25 PM, David Holmes <david.holmes at oracle.com> wrote:
> <replying on hotspot-dev>
> On 30/07/2015 1:28 AM, Christian Thalinger wrote:
>> Now that threadLS_solaris_sparc.{cpp,hpp} and threadLS_solaris_x86.{cpp,hop} look exactly the same it would be nice to merge them into threadLS_solaris.{cpp,hpp}.
> In the next phase:
> https://bugs.openjdk.java.net/browse/JDK-8132510
> these files will disappear completely. Can this wait till then?


> Thanks,
> David
>>> On Jul 28, 2015, at 10:56 PM, David Holmes <david.holmes at oracle.com> wrote:
>>> Moved to hotspot-dev so the compiler folk also see this for the MacroAssembler changes.
>>> David
>>> On 29/07/2015 3:53 PM, David Holmes wrote:
>>>> I forgot to credit Dave Dice with the suggestion to modernize this code.
>>>> David
>>>> On 29/07/2015 3:46 PM, David Holmes wrote:
>>>>> Summary: replace complex custom code for maintaining ThreadLocalStorage
>>>>> with compiler supported thread-local variables (Solaris only)
>>>>> This is a non-public bug so let me explain with some background, the
>>>>> bug, and then the fix - which involves lots of complex-code deletion and
>>>>> addition of some very simple code. :)
>>>>> webrev: http://cr.openjdk.java.net/~dholmes/8130212/webrev/
>>>>> In various parts of the runtime and in compiler generated code we need
>>>>> to get a reference to the (VM-level) Thread* of the currently executing
>>>>> thread. This is what Thread::current() returns. For performance reasons
>>>>> we also have a fast-path on 64-bit where the Thread* is stashed away in
>>>>> a register (g7 on sparc, r15 on x64).
>>>>> So Thread::current() is actually a slow-path mechanism and it delegates
>>>>> to ThreadLocalStorage::thread().
>>>>> On some systems ThreadLocalStorage::thread utilizes a caching mechanism
>>>>> to try and speed up access to the current thread. Otherwise it calls
>>>>> into yet another "slow" path which uses the available platform
>>>>> thread-specific-storage APIs.
>>>>> Compiled code also has a slow-path get_thread() method which uses
>>>>> assembly code to invoke the same platform thread-specific-storage APIs
>>>>> (in some cases - on sparc it simply calls ThreadLocalStorage::thread()).
>>>>> On Solaris 64-bit (which is all we support today) there is a simple
>>>>> 1-level thread cache which is an array of Thread*. If a thread doesn't
>>>>> find itself in the slot for the hash of its id it inserts itself there.
>>>>> As a thread terminates it clears out its ThreadLocalStorage values
>>>>> including any cached reference.
>>>>> The bug is that we have potential for a read-after-free error due to
>>>>> this code:
>>>>>   46   uintptr_t raw = pd_raw_thread_id();
>>>>>   47   int ix = pd_cache_index(raw);  // hashes id
>>>>>   48   Thread* candidate = ThreadLocalStorage::_get_thread_cache[ix];
>>>>>   49   if (candidate->self_raw_id() == raw) {
>>>>>   50     // hit
>>>>>   51     return candidate;
>>>>>   52   } else {
>>>>>   53     return ThreadLocalStorage::get_thread_via_cache_slowly(raw,
>>>>> ix);
>>>>>   54   }
>>>>> The problem is that the value read as candidate could be a thread that
>>>>> (after line 48) terminated and was freed. But line #49 then reads the
>>>>> raw id of that thread, which is then a read-after-free - which is a "Bad
>>>>> Thing (TM)".
>>>>> There's no simple fix for the caching code - you would need a completely
>>>>> different approach (or synchronization that would nullify the whole
>>>>> point of the cache).
>>>>> Now all this ThreadLocalStorage code is pretty old and was put in place
>>>>> to deal with inadequacies of the system provided thread-specific-storage
>>>>> API. In fact on Solaris we even by-pass the public API
>>>>> (thr_getspecific/thr_setspecific) when we can and implement our own
>>>>> version using lower-level APIs available in the T1/T2 threading
>>>>> libraries!
>>>>> In mid-2015 things have changed considerably and we have reliable and
>>>>> performant support for thread-local variables at the C+ language-level.
>>>>> So the way to maintain the current thread is simply using:
>>>>>  // Declaration of thread-local variable
>>>>>  static __thread Thread * _thr_current
>>>>>  inline Thread* ThreadLocalStorage::thread()  {
>>>>>    return _thr_current;
>>>>>  }
>>>>>  inline void ThreadLocalStorage::set_thread(Thread* thread) {
>>>>>    _thr_current = thread;
>>>>>  }
>>>>> And all the complex ThreadLocalStorage code with caching etc all
>>>>> vanishes!
>>>>> For my next trick I plan to try and remove the ThreadLocalStorage class
>>>>> completely by using language-based thread-locals on all platforms. But
>>>>> for now this is just Solaris and so we still need the ThreadLocalStorage
>>>>> API. However a lot of that API is not needed any more on Solaris so I
>>>>> have excluded it from there in the shared code (ifndef SOLARIS). But to
>>>>> avoid changing other shared-code callsites of ThreadLocalStorage I've
>>>>> kept part of the API with trivial implementations on Solaris.
>>>>> Testing: JPRT
>>>>>          All hotspot regression tests
>>>>> I'm happy to run more tests but the nice thing about such low-level code
>>>>> is that if it is broken, it is always broken :) Every use of
>>>>> Thread::current or MacroAssembler::get_thread now hits this code.
>>>>> Performance: I've run a basic set of benchmarks that is readily
>>>>> available to me on our performance testing system. The best way to
>>>>> describe the result is neutral. There are some slight wins, and some
>>>>> slight losses, with most showing no statistical difference. And even the
>>>>> "wins" and "losses" are within the natural variations of the benchmarks.
>>>>> So a lot of complex code has been replaced by simple code and we haven't
>>>>> lost any observable performance - which seems like a win to me.
>>>>> Also product mode x64 libjvm.so has shrunk by 921KB - which is a little
>>>>> surprising but very nice.
>>>>> Thanks,
>>>>> David

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