Getting the current thread ID without a syscall?
Konstantin Belousov
kostikbel at gmail.com
Tue Jan 15 21:43:38 UTC 2013
On Tue, Jan 15, 2013 at 04:35:14PM -0500, Trent Nelson wrote:
> 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.)
For you, may be. For your users, it definitely will be a problem.
And worse, the problem will be blamed on the operating system and not
to the broken application.
>
> > 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.
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