Re: spinlock requests (was RE: [Xen-devel] [Patch] don't spin with irq disabled)

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 31/03/2009 20:00, "Jeremy Fitzhardinge" <jeremy@xxxxxxxx> wrote:

>> If we care that much about fairness we should use ticket- or queue-based
>> locks. I don't believe any of our locks are contended enough to be a
>> concern. If they were, that would be a concern in itself.
> 
> Writer vs reader fairness in rwlocks is different from normal spinlock
> fairness.  One presumes that you're expecting to get multiple readers if
> you choose to use a rwlock, but that can end up excluding writers for an
> unbounded amount of time.

I suspect the existing uses of rwlock in Xen actually are because that
seemed a natural fit for the code -- obvious split between reader and writer
critical sections -- rather than because of excessive serialisation if using
a normal spinlock. I strongly disbelieve that lock acquire/release is a
significant performance bottleneck for us right now.

 -- Keir



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