Date: Tue, 15 Jan 2013 16:35:14 -0500 From: Trent Nelson <trent@snakebite.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: "freebsd-hackers@freebsd.org" <freebsd-hackers@freebsd.org> Subject: Re: Getting the current thread ID without a syscall? Message-ID: <20130115213513.GA53047@snakebite.org> In-Reply-To: <20130115211641.GC2522@kib.kiev.ua> References: <20130115205403.GA52904@snakebite.org> <20130115211641.GC2522@kib.kiev.ua>
next in thread | previous in thread | raw e-mail | index | archive | help
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.
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20130115213513.GA53047>