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

David Holmes david.holmes at oracle.com
Wed Jul 29 05:53:24 UTC 2015

I forgot to credit Dave Dice with the suggestion to modernize this code.


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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