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

Avi Kivity
2010-04-01T11:10:20Z

On 04/01/2010 01:31 PM, 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.
>

True. But this is a band-aid we can apply now while the correct fix is
being worked out.

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error compiling committee.c: too many arguments to function