On 04/03, Srivatsa Vaddagiri wrote:
>
> On Tue, Apr 03, 2007 at 07:03:36PM +0400, Oleg Nesterov wrote:
>
> > is better to introduce a new helper for that, kthread_thaw_stop() or
> > something.
>
> Will think of that.
I changed my mind :) The problem is general, I am starting to believe
it is better to change kthread_stop().
> > > kthread_stop(p)
> > > {
> > > int old_exempt_flags;
> > >
> > > task_lock(p);
> > > old_exempt_flags = p->flags;
> > > p->flags |= PFE_ALL; /* Exempt 'p' from being frozen? */
> >
> > I agree, we should mark this thread as non-freezable, but we can't modify
> > p->flags, this is racy. "current" owns its ->flags and it is not atomic.
> > Note that thaw_process() checks frozen(p) when it clears PF_FROZEN.
>
> I suspected that we cannot modify p->flags just like that. How abt
> moving freezer exemption bits to a separate field, which is protected by
> task_lock?
Probably yes... In that case it makes sense to move PF_FREEZER_SKIP/PF_FROZEN
to the new field as well.
Perhaps we can ignore this problem for now. Freezer is not 100% reliable
anyway. For example,
worker_thread:
for (;;) {
try_to_freeze();
prepare_to_wait();
if (...)
schedule();
finish_wait();
}
This is racy, we can miss freeze_process()->signal_wake_up() if it happens
between try_to_freeze() and prepare_to_wait(). We have to check TIF_FREEZE
before entering schedule() if we want to fix this race.
Should we? I don't know. This will uglify the code, and the probability
of this race is very low.
Oleg.
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