Re: How TLS is used in Kernel thread
- From: Doug Rabson <dfr@xxxxxxxxxx>
- Date: Sun, 02 Sep 2007 09:33:25 +0100
On Thu, 2007-08-30 at 14:20 -0700, Jin Guojun wrote:
By looking through _pthread_create() code and find it uses a magic
cookie -- TLS -- created
by rtld_allocate_tls(), and passed into kernel by sysarch() via
_tcb_set() / _kcb_set().
The information seems to be set by rtld (ld-elf.so.1) in digest_phdr()
under tag PT_TLS.
But it is very magic for where the TLS object is created and how it is
passed to digest_phdr().
The whole object passed into kernel (as sd.gsbase) looks like this:
TCB: ______________________________
| TLS | TCB |
|______________|_______________|
Can someone give some basic exaplain on following questions?
1) What TLS stand for?
Its used to implement Thread-Local-Storage variables. These are global
variables declared with the '__thread' keyword. Each variable declared
in this way has a per-thread value (i.e. each thread has a private copy
of the variable).
2) Where TLS object is created? (below is the tls assigned, but I couls
not find where ph is from)
case PT_TLS:
obj->tlsindex = 1;
obj->tlssize = ph->p_memsz;
obj->tlsalign = ph->p_align;
obj->tlsinitsize = ph->p_filesz;
obj->tlsinit = (void*) ph->p_vaddr;
Most of the work of dealing with TLS happens in the runtime
linker /libexec/ld-elf.so.1. The runtime linker calculates the TLS size
based on the TLS usage of all loaded libraries. The thread library uses
an internal interface to rtld (_rtld_allocate_tls and _rtld_free_tls)
during thread creation and destruction to allocate and free the TLS
blocks. As you have seen, the memory allocated also includes the
thread-library's control structures (which includes the saved register
set etc.).
Static programs have a slightly simpler mechanism (simpler because there
is only the main program and no dynamic libraries). This is handled in
src/lib/libc/gen/tls.c.
3) Where in kernel the TLS is used for thread operation?
The kernel doesn't use this information at all. It simply provides
support for the thread libraries needs.
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