Re: [PATCH] break_lock forever broken

[Nick Piggin]

Ingo Molnar wrote:
> * Hugh Dickins <hugh@xxxxxxxxxxx> wrote:
>
>
> > @@ -187,6 +187,8 @@ void __lockfunc _##op##_lock(locktype##_
> > 			cpu_relax();					\
> > 		preempt_disable();					\
> > 	}								\
> > +	if ((lock)->break_lock)						\
> > +		(lock)->break_lock = 0;					\
>
>
> while writing the ->break_lock feature i intentionally avoided overhead
> in the spinlock fastpath. A better solution for the bug you noticed is
> to clear the break_lock flag in places that use need_lock_break()
> explicitly.

What happens if break_lock gets set by random contention on the
lock somewhere (with no need_lock_break or cond_resched_lock)?
Next time it goes through a lockbreak will (may) be a false positive.

I think I'd prefer the additional lock overhead of Hugh's patch if
it gives better behaviour. I think. Any idea what the overhead actually
is?

> One robust way for that seems to be to make the need_lock_break() macro
> clear the flag if it sees it set, and to make all the other (internal)
> users use __need_lock_break() that doesnt clear the flag. I'll cook up a
> patch for this.

If you do this exactly as you describe, then you'll break
cond_resched_lock (eg. for the copy_page_range path), won't you?

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