Re: [RFC PATCH(Experimental) 0/4] Freezer based Cpu-hotplug

On Sunday, 18 February 2007 00:42, Oleg Nesterov wrote:
On 02/17, Rafael J. Wysocki wrote:

On Saturday, 17 February 2007 22:34, Oleg Nesterov wrote:

static inline int is_user_space(struct task_struct *p)
{
return p->mm && !(p->flags & PF_BORROWED_MM);
}

This doesn't look right. First, an exiting task has ->mm == NULL after
do_exit()->exit_mm(). Probably not a problem. However, PF_BORROWED_MM
check is racy without task_lock(), so we can have a false positive as
well. Is it ok? We can freeze aio_wq prematurely.

Right now aio_wq is not freezeable (PF_NOFREEZE).

Right now yes, but we are going to change this?

Well, is there any more reliable (and not racy) method of differentiating
between kernel threads and user space processes?

cancel_freezing(p);
continue;

Is it right? Shouldn't we increment "todo" counter?

No. It would be wrong to do that, because TASK_TRACED tasks with frozen
parents cannot be frozen any further.

TASK_TRACED task could be woken by SIGKILL. cancel_freezing() clears TIF_FREEZE.
The task may start do_exit() when try_to_freeze_tasks() returns "success".
Probably not a problem.

Yup.

Greetings,
Rafael
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