On Saturday, 12 May 2007 16:18, Oleg Nesterov wrote:
> On 05/12, Rafael J. Wysocki wrote:
> >
> > ... user space tasks that call deamonize() can also be frozen prematurely.
> > We didn't take this possibility into consideration before, which was
> > obviously
> > wrong.
>
> No, no, sorry for the confusion. User space tasks never call deamonize().
>
> Kernel threads call daemonize, because when we are doing kernel_thread()
> on behalf of user-space task, the new kernel thread (child) shares its
> ->mm with the caller (parent). So it is considered as "is_user_space()"
> until it does daemonize().
Ah, I see. We spawn a kernel thread from a code path that belongs to a
user space task and we need to call deamonize() to make it become a
'real' kernel thread.
Still, this means that is_user_space() may return 'true' for this thread
before it calls daemonize() and then the scenario described by me in the
previous message may occur. It seems.
> Definitely, is_user_space() should have another name.
Well, I have no (better) idea ...
> When a user space task exits, it does exit_mm() and becomes "a kernel thread"
> from the freezer POV. In its current from, freezer can do nothing with this.
> The exiting task won't call try_to_freeze() after that, so
> try_to_freeze_tasks()
> will wait until it dissapears (actually, until it calls exit_notify(), note
> the ->exit_state check in freezeable()).
>
> I do not think we can improve things if exit_mm() clears TIF_FREEZING.
I think we can avoid the above scenario (in which a kernel thread that has
called daemonize() "too late" ends up with TIF_FREEZE set and goes to
refrigerator() while we're freezing the user space).
> We should clear TIF_FREEZING when we set PF_NOFREEZE, I think. This was
> discussed before iirc, but I forgot the result.
It's in freezer-fix-pf_nofreeze-vs-freezeable-race.patch (appended for
convenience, white space may be broken).
Greetings,
Rafael
---
--- a/include/linux/freezer.h~freezer-fix-pf_nofreeze-vs-freezeable-race
+++ a/include/linux/freezer.h
@@ -63,8 +63,10 @@ static inline int thaw_process(struct ta
*/
static inline void frozen_process(struct task_struct *p)
{
- p->flags |= PF_FROZEN;
- wmb();
+ if (!unlikely(p->flags & PF_NOFREEZE)) {
+ p->flags |= PF_FROZEN;
+ wmb();
+ }
clear_tsk_thread_flag(p, TIF_FREEZE);
}
-
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