Re: [tip:locking/core] sched/wait: Fix signal handling in bit wait helpers

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Dec 10, 2015 at 08:30:01AM +1100, NeilBrown wrote:
> On Wed, Dec 09 2015, Peter Zijlstra wrote:
> 
> > On Wed, Dec 09, 2015 at 12:06:33PM +1100, NeilBrown wrote:
> >> On Tue, Dec 08 2015, Peter Zijlstra wrote:
> >> 
> >> >>  
> >> >
> >> > *sigh*, so that patch was broken.. the below might fix it, but please
> >> > someone look at it, I seem to have a less than stellar track record
> >> > here...
> >> 
> >> This new change seems to be more intrusive than should be needed.
> >> Can't we just do:
> >> 
> >> 
> >>  __sched int bit_wait(struct wait_bit_key *word)
> >>  {
> >> +	long state = current->state;
> >
> > No, current->state can already be changed by this time.
> 
> Does that matter?
> It can only have changed to TASK_RUNNING - right?
> In that case signal_pending_state() will return 0 and the bit_wait() acts
> as though the thread was woken up normally (which it was) rather than by
> a signal (which maybe it was too, but maybe that happened just a tiny
> bit later).
> 
> As long as signal delivery doesn't change ->state, we should be safe.
> We should even be safe testing ->state *after* the call the schedule().

Blergh, all I've managed to far is to confuse myself further. Even
something like the original (+- the EINTR) should work when we consider
the looping, even when mixed with an occasional spurious wakeup.


int bit_wait()
{
	if (signal_pending_state(current->state, current))
		return -EINTR;
	schedule();
}


This can go wrong against raising a signal thusly:

	prepare_to_wait()
1:	if (signal_pending_state(current->state, current))
		// false, nothing pending
	schedule();
				set_tsk_thread_flag(t, TIF_SIGPENDING);

		<spurious wakeup>

	prepare_to_wait()
				wake_up_state(t, ...);
2:	if (signal_pending_state(current->state, current))
		// false, TASK_RUNNING

	schedule(); // doesn't block because pending

	prepare_to_wait()
3:	if (signal_pending_state(current->state, current))
		// true, pending