ARM11MPcore: tlb_ops_need_broadcast causes deadlock

[Russell King - ARM Linux]

On Sun, Mar 25, 2012 at 08:22:05PM +0200, Peter Waechtler wrote:
> On 25.03.2012 15:09, Russell King - ARM Linux wrote:
>> On Sun, Mar 25, 2012 at 12:08:47PM +0000, Peter Waechtler wrote:
>>> But Will, is that tlb_flush necessary at all? The ARM has only 3 permission
>>> bits in the page table (APX and AP0 and AP1). The young/accessed bit is done
>>> via software.
>> Yes it most definitely is, because setting a page to be young means we
>> must receive a subsequent fault to make it 'old' again.  This means we
>> must set the page to be inaccessible to get that fault, and flush the
>> TLBs across all CPUs so that any CPU accessing that page receives a
>> fault.
> Ok I see, it's also not the "right or perfect" fix.

It's not a fix or anything, it's required behaviour - otherwise we could
end up throwing out pages from the system which are actually 'hot' because
they've stayed in the TLB and we haven't received a fault to make them
young again.

Moreover, what about the case where we actually remove the page?

Aren't we also holding the pte lock there?  So I don't think there's an
obvious solution to your deadlock.

I think the real question is - in your example - why are you touching
a userspace page with IRQs off _and_ expecting the fault to be fixed up?
You never really explained what CPU B was doing.