MMIO and gcc re-ordering issue

Chris Friesen cfriesen at
Wed May 28 07:23:51 EST 2008

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?


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