Re: [RFC][PATCH][kprobe] enabling booster on the preemptible kernel, take 2

[Masami Hiramatsu <masami dot hiramatsu dot pt at hitachi dot com>]

Hi Bibo,

bibo mao wrote:
> I am not familiar with freeze_processes(), I only view code.
> And I write simple program(though buggy) to test:
[...]
> it seems that many threads are not frozen even freeze_processes
> return 0.

I think the most important issue is whether those threads are
preempted (suddenly scheduled out) or not.
Those preempted threads might be preempted on the instruction
buffer or on the middle of the target instructions. In that
case, we cannot free the buffer or overwrite a jump code.
But, if those threads are sleeping at the known places, we can
ensure safety of freeing/overwriting.
Therefore, I think it is necessary to check whether all
non-frozen threads are sleeping or not, like as below;

---------------------------------------
int check_safety(void)
{
 int ret = 0;
 struct task_struct *g, *p;
 if (freeze_processes()) {
  goto Thaw;
 }
 do_each_thread(g, p) {
  if (frozen(p))
   continue;
  if (p->state == TASK_RUNNING) {
   ret = -EAGAIN;
   break;
  }
 } while_each_thread(g, p);
Thaw:
 thaw_processes();
 return ret;
}


Thanks,


-- 
Masami HIRAMATSU
Linux Technology Center
Hitachi, Ltd., Systems Development Laboratory
E-mail: masami.hiramatsu.pt@hitachi.com