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

David Holmes david.holmes at oracle.com
Wed Jul 29 05:56:08 UTC 2015

Moved to hotspot-dev so the compiler folk also see this for the 
MacroAssembler changes.


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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