(L) Prelim RFR: 8132510: Replace ThreadLocalStorage with compiler/language-based thread-local variables

Jeremy Manson jeremymanson at google.com
Fri Nov 6 02:24:20 UTC 2015

I should probably elaborate on this.  With glibc + ELF, the first time a
thread accesses a variable declared __thread, if that variable is in a
shared library (as opposed to the main executable), the system calls
malloc() to allocate the space for it.  If that happens in a signal that is
being delivered during a call to malloc(), then you usually get a crash.

This may bite you if AsyncGetCallTrace uses Thread::current(), and you use
system timers to do profiling.  If a thread doing a malloc() prior to the
first time it accesses Thread::current(), and it gets delivered a signal,
it might die.  This is especially likely for pure native threads started by
native code.

I believe that this is a use case you support, so you might want to make
sure it is okay.


On Thu, Nov 5, 2015 at 5:58 PM, Jeremy Manson <jeremymanson at google.com>

> Something that's bitten me with __thread: it isn't async-safe when called
> from a shared object on Linux.  Have you vetted to make sure this doesn't
> make HS less async-safe?
> Jeremy
> On Sun, Nov 1, 2015 at 10:40 PM, David Holmes <david.holmes at oracle.com>
> wrote:
>> bug: https://bugs.openjdk.java.net/browse/JDK-8132510
>> Open webrev: http://cr.openjdk.java.net/~dholmes/8132510/webrev.v2/
>> A simple (in principle) but wide-ranging change which should appeal to
>> our Code Deletion Engineer's. We implement Thread::current() using a
>> compiler/language-based thread-local variable eg:
>>  static __thread Thread *_thr_current;
>>  inline Thread* Thread::current() {
>>    return _thr_current;
>>  }
>> with an appropriate setter of course. By doing this we can completely
>> remove the platform-specific ThreadLocalStorage implementations, and the
>> associated os::thread_local_storage* calls, plus all the uses of
>> ThreadLocalStorage::thread() and ThreadLocalStorage::get_thread_slow().
>> This extends the previous work done on Solaris to implement
>> ThreadLocalStorage::thread() using compiler-based thread-locals.
>> We can also consolidate nearly all the os_cpu versions of
>> MacroAssembler::get_thread on x86 into one cpu specific one ( a special
>> variant is still needed for 32-bit Windows).
>> As a result of this change we have further potential cleanups:
>> - all the src/os/<os>/vm/thread_<os>.inline.hpp files are now completely
>> empty and could also be removed
>> - the MINIMIZE_RAM_USAGE define (which avoids use of the linux sp-map
>> "cache" on 32-bit) now has no affect and so could be completely removed
>> from the build system
>> I plan to do the MINIMIZE_RAM_USAGE removal as a follow up CR, but could
>> add the removal of the "inline" files to this CR if people think it worth
>> removing them.
>> I have one missing piece on Aarch64 - I need to change
>> MacroAssembler::get_thread to simply call Thread::current() as on other
>> platforms, but I don't know how to write that. I would appreciate it if
>> someone could give me the right code for that.
>> I would also appreciate comments/testing by the AIX and PPC64 folk as
>> well.
>> A concern about memory-leaks had previously been raised, but experiments
>> using simple C code on linux 86 and Solaris showed no issues. Also note
>> that Aarch64 already uses this kind of thread-local.
>> Thanks,
>> David

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