On 11/01, Tejun Heo wrote:
>
> Hello,
>
> On Tue, Nov 01, 2011 at 07:13:29PM +0100, Oleg Nesterov wrote:
> > On 11/01, Tejun Heo wrote:
> > >
> > > Yeah yeah, Trond already pointed it out. I forgot about the
> > > sigpending special case in schedule(), which I think is rather odd,
> >
> > I disagree with "rather odd" ;)
> >
> > We have a lot of examples of
> >
> > current->state = TASK_INTERRUPTIBLE;
> > ...
> > if (signal_pending())
> > break;
> > schedule();
> >
> > Without that special case in schedule() the code above becomes racy.
> > Just consider __wait_event_interruptible().
>
> But __wait_event_interruptible() does proper set-TASK_*, check
> sigpending and schedule() sequence. As long as the waker performs
> seg-sigpending, wakeup sequence in the correct order, nothing is
> broken (as w/ any other wakeup conditions). The special case deals
> with callers which don't check sigpending between set-TASK_* and
> schedule() and that's the part I think is a bit odd.
OK, agreed, __wait_event_interruptible() was a bad example.
Yes, this is only needed for the code which doesn't check
signal_pending() "properly", or doesn't check it at all before
schedule(). OK, say, wait_for_completion_interruptible().
Or schedule_timeout_interruptible().
I suspect we have a lot more examples. Historically linux allows to
set TASK_INTERRUPTIBLE and schedule() without any checks.
> Whether I feel
> odd or not is irrelevant tho - it's already there.
Yes, I don't think we can remove it.
> > > Any better ideas?
> >
> > Well. As a simple (probably temporary) fix, I'd suggest
> >
> > #define wait_event_freezekillable(wq, condition)
> > {
> > freezer_do_not_count();
> > __retval = wait_event_killable(condition);
> > freezer_count();
> > __retval;
> > }
> >
> > Do you think it can work?
>
> Yeah, probably. I was hoping to remove count/do_not_count tho.
Or at least rename it ;)
> Hmmm... maybe we can just flip PF_NOFREEZE instead with a bit of
> modification, I think.
Perhaps.
Or we can add TASK_FREEZABLE (like TASK_WAKEKILL), iirc we already
discussed this some time ago. And probably it makes sense to add the
generic wait_event_state().
Oleg.
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