Re: MMIO and gcc re-ordering issue

Chris Friesen
2008-05-27T21:24:20Z

Roland Dreier wrote:

> Writes are posted yes, but not reordered arbitrarily. If I have code like:
>
> spin_lock(&mmio_lock);
> writel(val1, reg1);
> writel(val2, reg2);
> spin_unlock(&mmio_lock);
>
> then I have a reasonable expectation that if two CPUs run this at the
> same time, their writes to reg1/reg2 won't be interleaved with each
> other (because the whole section is inside a spinlock). And Altix
> violates that expectation.

Does that necessarily follow?

If you've got a large system with multiple pci bridges, could you end up
with posted writes coming from different cpus taking a different amount
of time to propagate to a device and thus colliding?

Chris