* Linus Torvalds <[email protected]> wrote:
> > > I think it is better to set EXIT_DEAD in do_exit(), along with PF_DEAD
> > > flag.
> >
> > nice idea - your patch looks good to me.
>
> I'm not entirely convinced.
>
> The thing is, We used to have DEAD in the task state flags, ie
> TASK_ZOMBIE was it.
>
> We started using PF_DEAD in 2003 with this commit message:
>
> Author: Linus Torvalds <[email protected]> 2003-10-26 03:16:23
>
> Add a sticky "PF_DEAD" task flag to keep track of dead processes.
>
> Use this to simplify 'finish_task_switch', but perhaps more
> importantly we can use this to track down why some processes
> seem to sometimes not die properly even after having been
> marked as ZOMBIE. The "task->state" flags are too fluid to
> allow that well.
>
> ie the PF_DEAD flag was never really about is _needing_ it: it was all
> about being able to safely check it _without_ having to rely on
> task->state.
but this was in times when we did alot of nontrivial operations after we
marked a task "dead". Today we do this:
/* PF_DEAD causes final put_task_struct after we schedule. */
preempt_disable();
BUG_ON(tsk->flags & PF_DEAD);
tsk->flags |= PF_DEAD;
schedule();
BUG();
(i introduced the above changes to make more of the exit path
preemptable.)
PF_DEAD has zero relevance by today, and Oleg's patches are perfectly
correct and dont add the kind of risk that they'd have meant in 2003.
> So putting it back into task->state is not wrong per se, but it kind
> of misses the point of why it was somewhere else in the first place
> (or rather, why it was there in the _second_ place, since it was in
> task->state in the first place and got moved out of there).
yeah, PF_DEAD had its purpose back when we first marked a task dead,
then we did the release_task(). We used to have problems with
proc_pid_flush() which could sleep, which would lose the TASK_ZOMBIE or
TASK_DEAD flag and we'd come back from the 'final' schedule().
today that's impossible, due to the code above - we only mark it PF_DEAD
straight before going into the final schedule().
Ingo
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