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SubjectRe: [PATCH 2/2] sched: Introduce set_special_state()
On Mon, Apr 30, 2018 at 09:40:24PM +0200, Peter Zijlstra wrote:
> On Mon, Apr 30, 2018 at 06:45:46PM +0200, Oleg Nesterov wrote:
> > On 04/30, Peter Zijlstra wrote:
> > >
> > > --- a/kernel/signal.c
> > > +++ b/kernel/signal.c
> > > @@ -1968,7 +1968,7 @@ static void ptrace_stop(int exit_code, i
> > > * atomic with respect to siglock and should be done after the arch
> > > * hook as siglock is released and regrabbed across it.
> > > */
> > > - set_current_state(TASK_TRACED);
> > > + set_special_state(TASK_TRACED);
> >
> > Yes, but please note the comment above, we need a barrier after state = TASK_TRACED,
> > that is why ptrace_stop() does set_current_state(), not __set_current_state().
>
> OK, so I got properly lost in that stuff.
>
> The comment says it we need to set TASK_TRACED before clearing
> JOBCTL_TRAPPING because of do_wait(), but I cannot seem to locate code
> in do_wait() and below that cares about JOBCTL_TRAPPING.
>
> Could you elucidate my tired brain?

The only code I found that seems to care is ptrace_attach(), where we
wait for JOBCTL_TRAPPING to get cleared. That same function has a
comment about hiding the STOPPED -> RUNNING -> TRACED transition. So I'm
assuming it needs to observe TRACED if it observes !TRAPPING.

But I don't think there's enough barriers on that end to guarantee this.
Any ->state load after wait_on_bit() is, afact, free to have happened
before the ->jobctl load.

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