Linux中TLS

TLS（Thread Local Storage）

线程局部存储。

在Linux操作系统中，TLS保存成GDT中描述的一个段。

   1: /*

   2:  * This creates a new process as a copy of the old one,

   3:  * but does not actually start it yet.

   4:  *

   5:  * It copies the registers, and all the appropriate

   6:  * parts of the process environment (as per the clone

   7:  * flags). The actual kick-off is left to the caller.

   8:  */

   9: static struct task_struct *copy_process(unsigned long clone_flags,

  10:                     unsigned long stack_start,

  11:                     struct pt_regs *regs,

  12:                     unsigned long stack_size,

  13:                     int __user *child_tidptr,

  14:                     struct pid *pid,

  15:                     int trace)

  16: {

  17: ......

  18: retval = copy_thread(clone_flags, stack_start, stack_size, p, regs);

  19: ......

  20: }

   1: int copy_thread(unsigned long clone_flags, unsigned long sp,

   2:     unsigned long unused,

   3:     struct task_struct *p, struct pt_regs *regs)

   4: {

   5:     struct pt_regs *childregs;

   6:     struct task_struct *tsk;

   7:     int err;

   8:  

   9:     childregs = task_pt_regs(p);

  10:     *childregs = *regs;

  11:     childregs->ax = 0;

  12:     childregs->sp = sp;

  13:  

  14:     p->thread.sp = (unsigned long) childregs;

  15:     p->thread.sp0 = (unsigned long) (childregs+1);

  16:  

  17:     p->thread.ip = (unsigned long) ret_from_fork;

  18:  

  19:     task_user_gs(p) = get_user_gs(regs);

  20:  

  21:     p->thread.io_bitmap_ptr = NULL;

  22:     tsk = current;

  23:     err = -ENOMEM;

  24:  

  25:     memset(p->thread.ptrace_bps, 0, sizeof(p->thread.ptrace_bps));

  26:  

  27:     if (unlikely(test_tsk_thread_flag(tsk, TIF_IO_BITMAP))) {

  28:         p->thread.io_bitmap_ptr = kmemdup(tsk->thread.io_bitmap_ptr,

  29:                         IO_BITMAP_BYTES, GFP_KERNEL);

  30:         if (!p->thread.io_bitmap_ptr) {

  31:             p->thread.io_bitmap_max = 0;

  32:             return -ENOMEM;

  33:         }

  34:         set_tsk_thread_flag(p, TIF_IO_BITMAP);

  35:     }

  36:  

  37:     err = 0;

  38:  

  39:     /*

  40:      * Set a new TLS for the child thread?

  41:      */

  42:     if (clone_flags & CLONE_SETTLS)

  43:         err = do_set_thread_area(p, -1,

  44:             (struct user_desc __user *)childregs->si, 0);

  45:  

  46:     if (err && p->thread.io_bitmap_ptr) {

  47:         kfree(p->thread.io_bitmap_ptr);

  48:         p->thread.io_bitmap_max = 0;

  49:     }

  50:     return err;

  51: }

   1: /*

   2:  * Set a given TLS descriptor:

   3:  */

   4: int do_set_thread_area(struct task_struct *p, int idx,

   5:                struct user_desc __user *u_info,

   6:                int can_allocate)

   7: {

   8:     struct user_desc info;

   9:  

  10:     if (copy_from_user(&info, u_info, sizeof(info)))

  11:         return -EFAULT;

  12:  

  13:     if (idx == -1)

  14:         idx = info.entry_number;

  15:  

  16:     /*

  17:      * index -1 means the kernel should try to find and

  18:      * allocate an empty descriptor:

  19:      */

  20:     if (idx == -1 && can_allocate) {

  21:         idx = get_free_idx();

  22:         if (idx < 0)

  23:             return idx;

  24:         if (put_user(idx, &u_info->entry_number))

  25:             return -EFAULT;

  26:     }

  27:  

  28:     if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)

  29:         return -EINVAL;

  30:  

  31:     set_tls_desc(p, idx, &info, 1);

  32:  

  33:     return 0;

  34: }

   1: static void set_tls_desc(struct task_struct *p, int idx,

   2:              const struct user_desc *info, int n)

   3: {

   4:     struct thread_struct *t = &p->thread;

   5:     struct desc_struct *desc = &t->tls_array[idx - GDT_ENTRY_TLS_MIN];

   6:     int cpu;

   7:  

   8:     /*

   9:      * We must not get preempted while modifying the TLS.

  10:      */

  11:     cpu = get_cpu();

  12:  

  13:     while (n-- > 0) {

  14:         if (LDT_empty(info))

  15:             desc->a = desc->b = 0;

  16:         else

  17:             fill_ldt(desc, info);

  18:         ++info;

  19:         ++desc;

  20:     }

  21:  

  22:     if (t == &current->thread)

  23:         load_TLS(t, cpu);

  24:  

  25:     put_cpu();

  26: }

   1: static inline void fill_ldt(struct desc_struct *desc, const struct user_desc *info)

   2: {

   3:     desc->limit0        = info->limit & 0x0ffff;

   4:  

   5:     desc->base0        = (info->base_addr & 0x0000ffff);

   6:     desc->base1        = (info->base_addr & 0x00ff0000) >> 16;

   7:  

   8:     desc->type        = (info->read_exec_only ^ 1) << 1;

   9:     desc->type           |= info->contents << 2;

  10:  

  11:     desc->s            = 1;

  12:     desc->dpl        = 0x3;

  13:     desc->p            = info->seg_not_present ^ 1;

  14:     desc->limit        = (info->limit & 0xf0000) >> 16;

  15:     desc->avl        = info->useable;

  16:     desc->d            = info->seg_32bit;

  17:     desc->g            = info->limit_in_pages;

  18:  

  19:     desc->base2        = (info->base_addr & 0xff000000) >> 24;

  20:     /*

  21:      * Don't allow setting of the lm bit. It is useless anyway

  22:      * because 64bit system calls require __USER_CS:

  23:      */

  24:     desc->l            = 0;

  25: }

从上面的call_tree可以看到，在fork系统调用创建一个新的进程时，会为新的任务设置TLS。

参考：http://blog.csdn.net/dog250/article/details/7704898

fill_ldt设置GDT中第6个段描述符的基址和段限以及DPL等信息，这些信息都是从sys_set_thread_area系统调用的u_info参数中得来的。本质上，最终GDT的第6个段中描述的信息其实就是一块内存，这块内存用于存储TLS节，这块内存其实也是使用brk，mmap之类调用在主线程的堆空间申请的，只是后来调用sys_set_thread_area将其设置成了本线程的私有空间罢了，主线程或者其它线程如果愿意，也是可以通过其它手段访问到这块空间的。

因为TLS是一个对应于C/C++ Runtime库的概念，所以要深入了解TLS，需要结合glibc来理解。