Re: [PATCH 1/4, v2] x86: enlightenment for ticket spin locks - baseimplementation

From: Jeremy Fitzhardinge
Date: Wed Jun 30 2010 - 06:50:51 EST

On 06/30/2010 11:11 AM, Peter Zijlstra wrote:
>>> Uhm, I'd much rather see a single alternative implementation, not a
>>> per-hypervisor lock implementation.
>> How would you imaging this to work? I can't see how the mechanism
>> could be hypervisor agnostic. Just look at the Xen implementation
>> (patch 2) - do you really see room for meaningful abstraction there?
> I tried not to, it made my eyes bleed..
> But from what I hear all virt people are suffering from spinlocks (and
> fair spinlocks in particular), so I was thinking it'd be a good idea to
> get all interested parties to collaborate on one. Fragmentation like
> this hardly ever works out well.

Yes. Now that I've looked at it a bit more closely I think these
patches put way too much logic into the per-hypervisor part of the code.

> Ah, right, after looking a bit more at patch 2 I see you indeed
> implement a ticket like lock. Although why you need both a ticket and a
> FIFO list is beyond me.

That appears to be a mechanism to allow it to take interrupts while
spinning on the lock, which is something that stock ticket locks don't
allow. If that's a useful thing to do, it should happen in the generic
ticketlock code rather than in the per-hypervisor backend (otherwise we
end up with all kinds of subtle differences in lock behaviour depending
on the exact environment, which is just going to be messy). Even if
interrupts-while-spinning isn't useful on native hardware, it is going
to be equally applicable to all virtual environments.

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