Hello everyone..
I try to observe how TLS and NPTL work on kernel side. I use Redhat 9
with stock kernel 2.4.20-19.9 and glibc-2.3.2-11.9.
I create a simple pthread test program which forks 2 threads and simply
print "Hello world" message. At the end, I call pthread_join two times
to cleanly terminate these two threads and finally terminate the main
program.
Strace-ing on the program shows me these interesting outputs (i am sorry
if the line wrapping screws code pasted below):
mmap2(NULL, 8388608, PROT_READ|PROT_WRITE|PROT_EXEC,
MAP_PRIVATE|MAP_ANONYMOUS,
-1, 0) = 0x40037000
brk(0) = 0x8049824
brk(0x804a824) = 0x804a824
brk(0) = 0x804a824
brk(0x804b000) = 0x804b000
mprotect(0x40037000, 4096, PROT_NONE) = 0
clone(child_stack=0x408368d0,flags=CLONE_VM|CLONE_FS|CLONE_FILES|
CLONE_SIGHAND|CLONE_THREAD|CLONE_SETTLS|CLONE_PARENT_SETTID|
CLONE_CHILD_CLEARTID|CLONE_DETACHED,
[4954],{entry_number:6,base_addr:0x40836cc0,
limit:1048575,seg_32bit:1,contents:0, read_exec_only:0,
limit_in_pages:1,seg_not_present:0, useable:1}) = 4954
On the other hand, inside arch/i386/kernel/process.c, i found codes that
handles CLONE_SETTLS flag (copy_thread() function ):
if (clone_flags & CLONE_SETTLS) {
struct desc_struct *desc;
struct user_desc info;
int idx;
if (copy_from_user(&info, (void *)childregs->esi,
sizeof(info)))
return -EFAULT;
if (LDT_empty(&info))
return -EINVAL;
idx = info.entry_number;
if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
return -EINVAL;
desc = p->thread.tls_array + idx - GDT_ENTRY_TLS_MIN;
desc->a = LDT_entry_a(&info);
desc->b = LDT_entry_b(&info);
}
My questions is:
1. What is the purpose of allocating new LDT entry? Is it to provide new
segment for freshly created thread to store its thread-local data?
2. Is the format of LDT entry 100% similar with GDT entry? If yes, what
is the advantage of using LDT for this threading purpose instead of
GDT?
3. Observing strace results, it seems that the user space program
defines certain base address for new allocated segment along with other
attributes. On the kernel side, seems like these informations are
retrieved to construct new LDT entry. My early conclusions is: base
address is needed because thread local data (defined by __thread) only
contains offset, thus it needs base address to form a complete linear
address. Is this correct?
4. I read somewhere on the Internet that gs register is used to point on
the currently used LDT entry related with currently running thread. Is
this just a convention which is used inside the libpthread? or is there
other explanation on why using gs register for this purpose?
thank your for your help...again, sorry for this long e-mail.......
regards
Mulyadi
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