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

[David Holmes]

<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
>