Getting the current thread ID without a syscall?

[Trent Nelson]

On Tue, Jan 15, 2013 at 01:16:41PM -0800, Konstantin Belousov wrote:
> On Tue, Jan 15, 2013 at 03:54:03PM -0500, Trent Nelson wrote:
> > Howdy,
> > 
> >     I have an unusual requirement: I need to get the current thread ID
> >     in as few instructions as possible.  On Windows, I managed to come
> >     up with this glorious hack:
> > 
> > #ifdef WITH_INTRINSICS
> > #   ifdef MS_WINDOWS
> > #       include <intrin.h>
> > #       if defined(MS_WIN64)
> > #           pragma intrinsic(__readgsdword)
> > #           define _Py_get_current_process_id() (__readgsdword(0x40))
> > #           define _Py_get_current_thread_id()  (__readgsdword(0x48))
> > #       elif defined(MS_WIN32)
> > #           pragma intrinsic(__readfsdword)
> > #           define _Py_get_current_process_id() (__readfsdword(0x20))
> > #           define _Py_get_current_thread_id()  (__readfsdword(0x24))
> > 
> >     That exploits the fact that Windows uses the FS/GS registers to
> >     store thread/process metadata.  Could I use a similar approach on
> >     FreeBSD to get the thread ID without the need for syscalls?
> The layout of the per-thread structure used by libthr is private and
> is not guaranteed to be stable even on the stable branches.
> 
> Yes, you could obtain the tid this way, but note explicitely that using
> it makes your application not binary compatible with any version of
> the FreeBSD except the one you compiled on.

    Luckily it's for an open source project (Python), so recompilation
    isn't a big deal.  (I also check the intrinsic result versus the
    syscall result during startup to verify the same ID is returned,
    falling back to the syscall by default.)

> You could read the _thread_off_tid integer variable and use the value
> as offset from the %fs base to the long containing the unique thread id.
> But don't use this in anything except the private code.

    Ah, thanks, that's what I was interested in knowing.

> > 
> >     (I technically don't need the thread ID, I just need to get some
> >      form of unique identifier for the current thread such that I can
> >      compare it to a known global value that's been set to the "main
> >      thread", in order to determine if I'm currently that thread or not.
> >      As long as it's unique for each thread, and static for the lifetime
> >      of the thread, that's fine.)
> > 
> >     The "am I the main thread?" comparison is made every ~50-100 opcodes,
> >     which is why it needs to have the lowest overhead possible.

> On newer CPUs in amd64 mode, there is getfsbase instruction which reads
> the %fs register base. System guarantees that %fs base is unique among
> live threads.

    Interesting.  I was aware of those instructions, but never assessed
    them in detail once I'd figured out the readgsdword approach.  I
    definitely didn't realize they return unique values per thread
    (although it makes sense now that I think about it).

    Thanks Konstantin, very helpful.

        Trent.