Re: [COUNTERPATCH] mm: avoid overflowing preempt_count() in mmu_take_all_locks()

Thomas Gleixner
2010-04-01T11:04:50Z

On Thu, 1 Apr 2010, Peter Zijlstra wrote:

> I'm sure you dropped Ingo and Thomas by accident.
>
> On Thu, 2010-04-01 at 12:40 +0300, Avi Kivity wrote:
> > mmu_take_all_locks() takes a spinlock for each vma, which means we increase
> > the preempt count by the number of vmas in an address space. Since the user
> > controls the number of vmas, they can cause preempt_count to overflow.
> >
> > Fix by making mmu_take_all_locks() only disable preemption once by making
> > the spinlocks preempt-neutral.
>
> Right, so while this will get rid of the warning it doesn't make the
> code any nicer, its still a massive !preempt latency spot.

I'm not sure whether this is a real well done April 1st joke or if there
is someone trying to secure the "bad taste patch of the month" price.

Anyway, I don't see a reason why we can't convert those locks to
mutexes and get rid of the whole preempt disabled region.

Thanks,

tglx